I would never consider solvent for anything over 1" thick. Under 1" is fine. 2 part polymerizable cement is best for large projects over 1" due to the machining imperfections associated with thicker material and the difficulty in getting solvent to uniformly cover the area without bubbles. It's hard to get decent joints on big heavy panels with solvent which is why those in the know use the 2 part stuff for the big jobs. Look at how the seams are bonded in public tanks, these are done with 2 part cement and most are not annealed because they are assembled on site and the panels would not fit into an over. If you can anneal the panels after machining it's a good idea but not required. Curing with heat from 100-150F will produce a stronger joint also. All depends if the cement has a good bite on the base material and some will scuff sand to promote this. Much more to casting good joints than this simple summary, you have to think of #40/42 joint as a mechanical bond, solvent is more of a weld, if the weld dosen't penetrate the base metal good enough strength is jeopardized. Just like welding plate steel, i.e rivets.
The claim that 2 part cement is stronger? Controversial... but I believe the 2 part stuff is stronger. There is a lot of data and practical evidence to support this if you do the research.
