Just a quick bit of info to share.

Most epoxy paint (the two part stuff) is simply a thin fiberglass resin. The only difference when you do fiberglass is that you add fiberglass cloth and then spread the resin on top. (of course there is a bit more skill required). The idea behind the cloth is that it adds strength to your project.

I used to work making houseboats and doing the fiberglass parts. The difference between that and a sink is mostly the smooth finish. A boat has to beautiful to sell where as a sink only has to be functional. I have recently (2 months ago) made a plywood/fiberglass sink for my darkroom and am just doing the plumbing changes over the next two weeks.

I suggest that anybody who wants to waterproof the sink obtain a beginner's book to fiberglassing from the library and read a bit. All that you really need to do is have fresh air, mix the epoxy resin /catalyst and lay the stuff down. (a bit simple but it actually works). The paint stuff that sells is usually more diluted (easier to spread) and sometimes they give you some grit to add so that it is not as "slick as grease" when it gets wet.

I would not place caulking under fiberglass resin. I would do all my resin first, let the resin cure and then if necessary do caulking.

In my sink I did not require any caulking, the resin sealed everything. Even though as mentioned it does not have to hold water like a bathtub I tested mine anyways. I filled it with water in the garage and left it to sit overnight.

Watertight and nary a leak. :P

Kind Regards,