Underwater can be easy, and it can suck (to be blunt). This was a pain. Process is the same as out of the water. Think of the acylic like wood - and you need to sand it smooth. I start with an 800 sandpaper and work my way down. On the inside, I used a polishing pad as the final step. If it were dry, I'd use an acrylic polish (3M makes some good stuff, so does Novus).This is totally off-track, so maybe you can direct to another thread if it already exists, but I am curious to learn more about buffing and polishing acrylic while the tank is filled -- ie, what steps are involved as opposed to a buff/polish on an empty tank? I'd love to hear more about doing both the exterior and the interior work!
Really can't argue with those results, fantastic work.
The one thing you get to do outside that is sometimes not an option inside is use tools. Small scratches I do by hand just out personal choice. Using a polishing wheel (even if it's on a drill bit) helps alot.
Underwater, you can use pneumatic tools (air powered). I tried on this, but couldn't. The bubbles coming off the drill where so much it was splashing water over the edge of the tank.
Step One: Mark off the area around the scratch.
Step Two: Start with your heavier sandpaper and sand the whole area smooth (not just the scratch - you need to blend it away).
Step Three: Go to a finer grade paper.
Step Four: Yep, an even finer grade paper.
Step... well you get the idea. Sand until it's smooth and just lightly hazed over. Make sure to blend the entire area.
Last step: Use a polishing pad/buffing pad to really shine it up.
I'll take some pics of the stuff tomorrow when I'm at work to give you an idea of what the stuff looks like.