epoxy? liquid rubber? maybe both???

[jjohnwm]

We have plenty of discussions about the strong points and weak points of two major schools of thought here: epoxy-type wood sealants versus liquid rubber products. I have only used epoxy. My older tanks (30-ish years ago...) were sealed with a supertough epoxy made by Pratt and Lambert, called PalGuard. I invariably used multiple thin coats, applied by brush, with no thinning of the initial coating. The wood was completely sealed, and the sealant allowed to cure thoroughly, before the glass was finally siliconed in place. I never had a problem with a leaking seal around the glass, and the silicone adhered to the epoxy perfectly.

The very few leaks I did run up against were in random locations on interior surfaces, usually on seams. Plenty of threads seem to link this type of leak to "micro-fractures" in the epoxy coating; these are often the same threads where a tank is built in the garage or workshop, and then...after the sealant is applied, and often after the glass is installed...the entire heavy, bulky construction is wheeled out of the garage, down the driveway, up the walkway, through the front door, down the hall, down a series of stairs and across the basement floor to its final home, where three men and a boy struggle it up into position on its stand. The inevitable flexing and jarring that all this movement creates simply must contribute to the failure of the epoxy coating to maintain a continuous, waterproof seal. I think that the answer is to build the tank in place, or at least move it into place before sealing the wood and installing the glass. Thoughts?

Liquid rubber is appealing in some ways, but the difficulty of using it with silicone for installation of the front glass has prevented me from trying it. Silicone doesn't stick to rubber...rubber doesn't stick to glass...and then the next thread says that's all wrong, it works great! I just finished epoxy-coating the interior of a 360gallon plywood tank, and I'm toying with the idea of adding a coat or two of rubber on top of the epoxy as an insurance against micro-fractures. Best of both worlds? My most recent build, a couple years back, used Pond Shield, and this current one uses an Interseal product that seems to check off all the same boxes while costing far, far less. Is the addition of a surface skin of liquid rubber overkill? Not that there's anything wrong with overkill... Thoughts?

This new build is also my first attempt at adhering glass directly to bare wood, using silicone. A few experiments have shown that this should work. I have the glass glued into place, with the silicone acting strictly as an adhesive at this point. Gave it a couple weeks to completely cure, then stood the tank up into final position and applied epoxy to all inner surfaces. I'm giving it a couple more weeks for epoxy cure, and then will run another thick silicone bead around the edges of the glass, such that the new bead will completely bridge the tiny gap between the glass and the edge of the epoxy paint. This second bead will be strictly a sealant, rather than an adhesive. The tank was never budged after the first coat of epoxy was applied. Thoughts?

Painfully slow? Absolutely! But one thing I have definitely learned from previous builds is to read the manufacturer's information on the various products you use...and then actually follow their recommendations. If they say it will take 1-2 weeks for curing...wait at least 2 weeks. If they say to apply it thick, or thin, or a specific way...then do that! "Can't wait...can't wait...CAN'T WAIT!!!!" is the wrong approach. "This might work...let's try it!" is definitely the wrong approach if you are doing it on the actual tank, rather than a test piece of wood or glass as an experiment.

Thoughts?

 

[frasertheking]

We have plenty of discussions about the strong points and weak points of two major schools of thought here: epoxy-type wood sealants versus liquid rubber products. I have only used epoxy. My older tanks (30-ish years ago...) were sealed with a supertough epoxy made by Pratt and Lambert, called PalGuard. I invariably used multiple thin coats, applied by brush, with no thinning of the initial coating. The wood was completely sealed, and the sealant allowed to cure thoroughly, before the glass was finally siliconed in place. I never had a problem with a leaking seal around the glass, and the silicone adhered to the epoxy perfectly.

The very few leaks I did run up against were in random locations on interior surfaces, usually on seams. Plenty of threads seem to link this type of leak to "micro-fractures" in the epoxy coating; these are often the same threads where a tank is built in the garage or workshop, and then...after the sealant is applied, and often after the glass is installed...the entire heavy, bulky construction is wheeled out of the garage, down the driveway, up the walkway, through the front door, down the hall, down a series of stairs and across the basement floor to its final home, where three men and a boy struggle it up into position on its stand. The inevitable flexing and jarring that all this movement creates simply must contribute to the failure of the epoxy coating to maintain a continuous, waterproof seal. I think that the answer is to build the tank in place, or at least move it into place before sealing the wood and installing the glass. Thoughts?

Liquid rubber is appealing in some ways, but the difficulty of using it with silicone for installation of the front glass has prevented me from trying it. Silicone doesn't stick to rubber...rubber doesn't stick to glass...and then the next thread says that's all wrong, it works great! I just finished epoxy-coating the interior of a 360gallon plywood tank, and I'm toying with the idea of adding a coat or two of rubber on top of the epoxy as an insurance against micro-fractures. Best of both worlds? My most recent build, a couple years back, used Pond Shield, and this current one uses an Interseal product that seems to check off all the same boxes while costing far, far less. Is the addition of a surface skin of liquid rubber overkill? Not that there's anything wrong with overkill... Thoughts?

This new build is also my first attempt at adhering glass directly to bare wood, using silicone. A few experiments have shown that this should work. I have the glass glued into place, with the silicone acting strictly as an adhesive at this point. Gave it a couple weeks to completely cure, then stood the tank up into final position and applied epoxy to all inner surfaces. I'm giving it a couple more weeks for epoxy cure, and then will run another thick silicone bead around the edges of the glass, such that the new bead will completely bridge the tiny gap between the glass and the edge of the epoxy paint. This second bead will be strictly a sealant, rather than an adhesive. The tank was never budged after the first coat of epoxy was applied. Thoughts?

Painfully slow? Absolutely! But one thing I have definitely learned from previous builds is to read the manufacturer's information on the various products you use...and then actually follow their recommendations. If they say it will take 1-2 weeks for curing...wait at least 2 weeks. If they say to apply it thick, or thin, or a specific way...then do that! "Can't wait...can't wait...CAN'T WAIT!!!!" is the wrong approach. "This might work...let's try it!" is definitely the wrong approach if you are doing it on the actual tank, rather than a test piece of wood or glass as an experiment.

Thoughts?

Epoxy would only really be under stress at the seams which is mostly because people do not use epoxy fillets so the stress is all in the corner. Epoxy is also mainly good in compression, the fibers give it the tensile strength. You do not have to glass everything, tape works in the high stress areas such as the corners but a couple of layers of glass is easy and you can do fun stuff like add dyes to the epoxy without much lose of strength or any really and have a deep colour with depth going all the way through your cloth.

Thinning the epoxy is not really a good idea for a few reasons
1. The epoxy is no longer waterproof
2. The epoxy loses 20-50% of its strength, depends on the thinner and how much
3. the epoxy becomes much more elastic so it will not add strength in the layers. If the tank ever did move the bottom layer with resist movement less and stretch more so the epoxy coating will not act as a cohesive layer.

This is my understanding of it anyway. The bonding strength of raw epoxy is great with good preparation and the entire composite is only as strong as the shear strength between the plywood layers anyway.

I have never tried liquid rubber but epoxy seems so perfect and if your willing you can find chinesium grade stuff with 90% of the properties for £50 a gallon if you search about. here in the Uk I get 6 liters of marine grade epoxy for £80. If you have a look at the thread I made on the subject there is a free book that details the basics of boat construction with epoxy and wood.

One thing that is universal with construction epoxy is it will benefit from a secondary cure at elevated temperatures. Slowly raising it to 40c or so for a few hours then slowly bringing it back down will increase the strength 10% or more but you lose a bit of the flexibility. Infact some really high end epoxies will cure at room temp and then must be heated in a controlled manner like this and have some insane properties.

Thinning epoxy: Best practice for reducing viscosity

Thinning epoxy can make it easier to use and help saturate materials. Discover ways to thin epoxy and considerations before you get started.

epoxycraft.com

 

[fishdance]

The biggest drawback of epoxy on wood is that epoxy is stiff and inflexible whilst wood moves. Simply draining a deep wooden tank (1.5m) can create stress cracks as tanks can gain a "memory" over time. It's somewhat misleading to rate tanks by size or volume as depth is the critical factor for these type of discussions. Some potential issues can be solved by design (such as using a steel frame) or understanding /limiting water depth. Other issues can be solved by technique (such as multiple layers, different colours to spot holes, experience, etc). Luckily technology improves constantly so choice and greater expectations evolve. Unfortunately people like to push limits.

There isn't a right or a wrong way to build a tank. Besides wood, concrete, sheet steel, HDPE, PVC, fiberglass can be used. On big tanks, it's not just the waterproofing but the safety aspect. Even a 5000L tank letting go could kill someone.

One thing to keep in mind that whatever advice is given now may not apply in a years time.

 

[jjohnwm]

Thanks to both F frasertheking and F fishdance for your comments.

The flexing associated with simply raising and lowering the water level a significant amount is something about which I have been thinking for awhile now; never have enough things to worry about, I guess. The lack of flexibility seems to be a non-issue for tanks of the size I build (depths of 24 inches), but even so, those dreaded "micro-fractures" are a disturbing thought. I have my own ideas about how to build my tanks, based upon the simple fact that I've made a bunch over the years that worked wonderfully...but I'd definitely be re-examining everything I "know" if I started building true monster-sized (i.e. DEEP!) tanks. It's not that the advice or methodology becomes less valid over time, but rather that the challenges of dealing with the much higher pressure of the deep water require more care in design and execution.

The three "downfalls" of thinning the first epoxy coat (first coat only!) were discussed with a couple of the companies whose epoxies I was considering...and they didn't seem to consider them downfalls, but merely considerations. Thinning only the first layer allows it to penetrate the wood more effectively, making the top wood layer stronger and also providing a more perfect adhesion with successive layers. The possibility of slightly increased flexibility seems to be a positive rather than negative, in terms of preventing micro-fractures.

The idea of "strength" of the epoxy is curious. What kind of strength is required? I need it to be hard enough to resist the inevitable bumps and bruises from rocks, etc. but it needs to have a minute degree of elasticity to maintain a seal when the tank inevitably flexes with changes in water pressure...or perhaps when it is lifted or moved. I suppose its strength would come into question if it were used as the resin in combination with fibreglass cloth, as some builders seem to do...but I have never used any fibreglass cloth anywhere in my tanks. A properly-assembled box of 3/4-inch plywood, glued and screwed, is easily strong enough to contain the water pressure in my 24-inch deep tanks, so why bother with the glass cloth?

The build I just finished went smoothly, and the entire time I was talking myself into putting a layer of liquid rubber on top of the epoxy. When I finished the epoxy and the time had come...I just said no. Never needed it before, so why make more work for myself? I enjoy looking at the completed project and knowing I did it myself; don't care too much for the actual process doing it myself...

 

[frasertheking]

Thanks to both F frasertheking and F fishdance for your comments.

The flexing associated with simply raising and lowering the water level a significant amount is something about which I have been thinking for awhile now; never have enough things to worry about, I guess. The lack of flexibility seems to be a non-issue for tanks of the size I build (depths of 24 inches), but even so, those dreaded "micro-fractures" are a disturbing thought. I have my own ideas about how to build my tanks, based upon the simple fact that I've made a bunch over the years that worked wonderfully...but I'd definitely be re-examining everything I "know" if I started building true monster-sized (i.e. DEEP!) tanks. It's not that the advice or methodology becomes less valid over time, but rather that the challenges of dealing with the much higher pressure of the deep water require more care in design and execution.

The three "downfalls" of thinning the first epoxy coat (first coat only!) were discussed with a couple of the companies whose epoxies I was considering...and they didn't seem to consider them downfalls, but merely considerations. Thinning only the first layer allows it to penetrate the wood more effectively, making the top wood layer stronger and also providing a more perfect adhesion with successive layers. The possibility of slightly increased flexibility seems to be a positive rather than negative, in terms of preventing micro-fractures.

The idea of "strength" of the epoxy is curious. What kind of strength is required? I need it to be hard enough to resist the inevitable bumps and bruises from rocks, etc. but it needs to have a minute degree of elasticity to maintain a seal when the tank inevitably flexes with changes in water pressure...or perhaps when it is lifted or moved. I suppose its strength would come into question if it were used as the resin in combination with fibreglass cloth, as some builders seem to do...but I have never used any fibreglass cloth anywhere in my tanks. A properly-assembled box of 3/4-inch plywood, glued and screwed, is easily strong enough to contain the water pressure in my 24-inch deep tanks, so why bother with the glass cloth?

The build I just finished went smoothly, and the entire time I was talking myself into putting a layer of liquid rubber on top of the epoxy. When I finished the epoxy and the time had come...I just said no. Never needed it before, so why make more work for myself? I enjoy looking at the completed project and knowing I did it myself; don't care too much for the actual process doing it myself...

West Coast are good with community stuff and they are worth speaking to about the thinning of epoxy. For me it was more in line with advice for building boats but they will transfer over. You can build a great tanks with screws and just epoxy sure but if you add epoxy fillets you can double the strength of your joints pretty much, adding tape over the fillets on just the corners will again make the entire structure stronger and far more resistant to the flexing that can cause problems. A couple of layers of light cloth will increase the stiffness of the wood sheet hugely which again prevents the microfractures that can form. Even without glassing the entire thing for 15 dollars of cloth tape and 10 dollars of thickener you can increase your projects strength by a huge margin without as I see any major downsides while learning a new skill.

epoxy is a fun material to mess with because it becomes less building with parts and more of a monocoque design acting as a single unit. This is how epoxy is used to create structures with a greater strength to weight ratio than many metals. I have never built a monster tank myself so it is possible mines would fail will yours would not but I think if we learn from each other we all get better.

 

[fishdance]

I don't think you will have a problem if you stick with what you are used to.

On 60cm water depth tanks, it is easiest to make an all glass tank.

Clear epoxy would show wood grain nicely though.

 

[jjohnwm]

It's also interesting to see what types of micro-fractures we are talking about. I had always assumed...and many articles seemed to confirm...that these microscopic cracks would be occurring at various points on the surface of the wood, caused by minute flexing created when adding/removing water (i.e. changing pressure). One source even referred to pressure differentials created by someone climbing into the empty tank during construction and concentrating his/her weight on two small footprints rather than spreading it out. The bottom of the tank appears well protected from this problem, since it is supported by numerous crossbraces in the top of the stand. It seems that a tiny amount of this type of "drumming" would be inevitable, especially in the large single piece of wood constituting the back of the tank, regardless of how much stiffness is added by fibreglass reinforcement. But...how much? Enough to create fractures?

But now I am reading about microfractures occurring along the corner seams. Hmmm...I hadn't considered that until now; frankly, I have a hard time buying it. My tanks, and most others I believe, are constructed as solid boxes with the front window opening cut out of one solid piece of wood constituting the front. After the window pane is installed, all top edges are braced solidly, creating a very rigid shell. Each interior seam is locked three-dimensionally in position by two adjacent sides at right angles. Personally, I also take great pains not to move a completed tank , as I feel that this is probably the factor most likely to create stresses and possible fractures. Finally, a thick bead of silicone is run along all interior seams as insurance against tiny holes or gaps in the epoxy, and this would also seal and possible microfractures running along the length of the seam.

I think that it's important to bear in mind the size of the tanks in question. I suspect that a tank of ~360 gallons (8x3x2 feet) has much more in common with a typical 50-gallon tank than it does with a true "monster" tank that is 3 or 4 or more feet in depth. Once you get into those realms...which I have not done!...a lot of these non-factors will quickly start to become much more than merely theoretical. If I were to attempt one of those, I would be applying a lot of these ideas, along with many others which I think are frivolous overkill for my "mid-range" tanks.

In a way, I'm actually glad that I don't live in one of those mild climates that encourage poolside tanks, outdoor patio ponds and other tropical luxuries. I'd be running down the path towards real monster tanks...a pathway lined with financial ruin, divorce, strained backs, torn ligaments...yeah, all that fun stuff...

 

[frasertheking]

It's also interesting to see what types of micro-fractures we are talking about. I had always assumed...and many articles seemed to confirm...that these microscopic cracks would be occurring at various points on the surface of the wood, caused by minute flexing created when adding/removing water (i.e. changing pressure). One source even referred to pressure differentials created by someone climbing into the empty tank during construction and concentrating his/her weight on two small footprints rather than spreading it out. The bottom of the tank appears well protected from this problem, since it is supported by numerous crossbraces in the top of the stand. It seems that a tiny amount of this type of "drumming" would be inevitable, especially in the large single piece of wood constituting the back of the tank, regardless of how much stiffness is added by fibreglass reinforcement. But...how much? Enough to create fractures?

But now I am reading about microfractures occurring along the corner seams. Hmmm...I hadn't considered that until now; frankly, I have a hard time buying it. My tanks, and most others I believe, are constructed as solid boxes with the front window opening cut out of one solid piece of wood constituting the front. After the window pane is installed, all top edges are braced solidly, creating a very rigid shell. Each interior seam is locked three-dimensionally in position by two adjacent sides at right angles. Personally, I also take great pains not to move a completed tank , as I feel that this is probably the factor most likely to create stresses and possible fractures. Finally, a thick bead of silicone is run along all interior seams as insurance against tiny holes or gaps in the epoxy, and this would also seal and possible microfractures running along the length of the seam.

I think that it's important to bear in mind the size of the tanks in question. I suspect that a tank of ~360 gallons (8x3x2 feet) has much more in common with a typical 50-gallon tank than it does with a true "monster" tank that is 3 or 4 or more feet in depth. Once you get into those realms...which I have not done!...a lot of these non-factors will quickly start to become much more than merely theoretical. If I were to attempt one of those, I would be applying a lot of these ideas, along with many others which I think are frivolous overkill for my "mid-range" tanks.

In a way, I'm actually glad that I don't live in one of those mild climates that encourage poolside tanks, outdoor patio ponds and other tropical luxuries. I'd be running down the path towards real monster tanks...a pathway lined with financial ruin, divorce, strained backs, torn ligaments...yeah, all that fun stuff...

In boats with just an epoxy coating checking is also a problem with just epoxy coating which glass fibre prevents. These are boat problems that have a lot of abuse compared to a tank sitting still. You can for sure build a tank with just epoxy and since most tanks are build with engineered woods like ply they are probably a lot more stable that a boat made of planks. Still when it comes to glass it might add 20-30% to the cost but the tank should in theory should last 50 years or more if you precoat all the wood edges in epoxy before putting it together. With marine plywood this is needed for the warranty which is usually 20 years. For most monster tanks they are not really easy to move and sell so it might not bother most builds.

 

[fishdance]

Can I jump in with a question fraserking? Sorry to detract from the origional thread John.

I'm looking at building a tank 1.2m water depth. Steel framed and fiberglassed ply wood seems to be the most common & reliable method. However, I'm not a huge fan of wood. Would fiberglass over 40mm or 60mm thick polystyrene (can add ribs) be just as strong? I could coat the polystyrene with multiple layers of acrylic paint to stop the fiberglass melting the polystyrene...

 

[JasperEllington]

In boats with just an epoxy coating checking is also a problem with just epoxy coating which glass fibre prevents. These are boat problems that have a lot of abuse compared to a tank sitting still. You can for sure build a tank with just epoxy and since most tanks are build with engineered woods like ply they are probably a lot more stable that a boat made of planks. Still when it comes to glass it might add 20-30% to the cost but the tank should in theory should last 50 years or more if you precoat all the wood edges in epoxy before putting it together. With marine plywood this is needed for the warranty which is usually 20 years. For most monster tanks they are not really easy to move and sell so it might not bother most builds.

That’s a great point about epoxy checking issues on boats—constant flexing and exposure to moisture can cause problems that a stationary tank might not face. For marine applications, fiberglass reinforcement over epoxy is a common practice to prevent structural failures, especially in hulls that endure significant stress. Marine surveys often reveal cases where improper sealing or material choice leads to long-term water ingress, reducing the vessel’s lifespan. For anyone considering long-term durability, applying additional layers of protection—whether it's fiberglass over epoxy or even reinforcing with liquid rubber—can extend longevity, much like ensuring a boat meets Boat Condition Survey standards before hitting the water.