Cost considerations
As far as I know, most allboats are made of polester resin, becuase this is the cheapest way to go. Also there is less chance of getting the resin to no set properly by incomplete or inadequate amount/activity of catalyst. As well as increasing brittlenes as the resin ages (those stress cracks?) the only real drawback to polyester is the fact that it does absorb water. I saw that when I had my emergency rudder (made of marine ply) initially coated in polyester resin and not eposxt pant, as now. When left in the water for a fe days, the resin went "milky". This, of course, is the reason for gecloat blistering under the waterline and why an EPOXY barrier coat is so essential. However, I do understand that some newer boats (can't recollect which ones) use vinyl ester resins below the waterline. This stuff is much less water absorbent and the basis of one of 3M's "blister repairs". With regards to adhesion of polyester to cured polyester - this is done all the time in boat repairs and proper keying is important as is application some solvent like acetone that not only cleans but softens the resin underneath to help with chemical bonding. Epoxy DOES stick well, however. That is why the strongest glues are often epoxies (used in aiplane manufacture). ALso, when restoring old cars, always apply the POLYESTER body filler over an epoxy primer coat if you never want rust (remember polyester is porous) or lack of adhesion. What a ramble!! Thanks Jim W
