What is the cheapest foam sheeting that one can buy that will not
melt when you apply polyester resin to it? What is the best source for
this foam? How much does it cost? Would it better than using luan as a
core?

I need a bigger jon boat. Mine is 40 years old and starting to fall
apart. I am going to build one, but money is a deciding factor in
this. If I had the cash, I would just go to Outdoor World and buy one
of theirs for $500. But since I don't have that kind of green I guess
I can build one. I would like to use foam as a core because it is
light and easy to form. I could see glueing a boat together out of 1/2
or 3/4 inch sheet foam and then glassing it. It just feels like it
would be easier than building a frame and then attaching the luan and
then glassing that. However I have never built a boat. I have rebuilt
a couple, but never built one from scratch.

Any advice? Pointers? Suggestions?