Hi Erez;

Sorry, I thought you'd read my (extensive) posts on this over the last month or so, as I've been a "guineau pig," testing a PR final finish - Last report due soon.

First off, I'm not entirely sure of the makeup of gelcoat. I can only speak to the PR stuff. PR is a 2 part mix, the resin and a catalyst. Lots of resin, drops of catalyst. The more catalyst, the faster the cure time. PR is what has historically always been used for fiberglassing (think Corvette body).

PR will never dry hard if oxygen can get to the surface. This facilitates extra coats because the next coat chemically bonds to the previous surface (within a reasonable time frame as I understand it, just like epoxy). A chemical bond is one where the molecules connect to each other via the valence electrons in the atoms themselves, in other words, the 2 coats become essentially one. This is a great bond. A mechanical bond relies on the roughness of the first coat (probably sanded) allowing the new coat to simply "grab on" tightly to all the pits, valleys, points etc. and harden in that form. This is a good bond, like varnish on epoxy.

Since PR doesn't cure hard when exposed to oxygen, we add a wax/styrene mix to it for the FINAL coat which rises to the surface as it cures and blocks contact with air. This leaves the last coat very (VERY) hard for sanding and buffing. I can tell you that stuff is hard by the number of sanding disks in my garbage pail . . .

Hope this helps, Spidey