I want to make some body mods on my 87 GT and would like to make one time disposable molds out of foam. I would like to use polyester resin , but I don't know what foam to use. Any help out there?

Urethane based foam will work. But styrene based foam (i.e. Styrofoam) will not. You should be able to buy blocks and slabs of urethane foam at crafts stores, or online industrial supply houses. You can also get it in a spray can at your local hardware store (the "Great Stuff" brand). And you can get it in liquid form (2 parts that you mix together) at most fiberglass supply stores.

yup - Great Stuff works well.
I just used it to fill in the gaps of a Dodge Shadow wing.
goo'd it up with the Great Stuff, cut down to shape, then fiberglassed over it
then used Bondo to smooth it out.

I have a lot of experience with this. All foams will degrade/shrink/distort when the FRP resin kicks off (the hardening action creates heat), some are destroyed by the resin itself.
I don't know what you are intending, but preformed urethane based foam blocks are the best way to go. They can be easily formed and urethane glue will bond them to panels and together.
The great stuff is an expanding foam. It will distort panels as it expands (if it get between panels).
Dap makes a minimal expansion white foam, but will distort heavily with the resin. It can still distort panels, but not nearly like Great Stuff will.

Some foam forms can be coated with wax paper or duct tape to lessen the deformation, but this depends on how you plan to make your panels / parts.

-Dave

Adding: Use the fiberglass Mat to build up the panel. Use as little filler as possible to finish the part. It will be stronger.

Or use whatever foam you want and use epoxy resin instead. Epoxy will not eat foam, smell, or dissolve your lungs like the cheap polyester resins. Epoxy is stiffer when cured as well. West Marine makes a great epoxy resin system.

one time cheap molds = plaster

for bigger parts use wood and burlap backing on the other side
donot try to go too thin it will crack
and wax it heavily

for low production fiberglass itself is hard to beat as a mold material

now the buck the mold is made from [ the shape your are molding from ie the ''model part'']
can be anything plaster again clay wax urethane foam cardboard balsa or other wood ect
BUT a VERY GOOD SURFACE is needed to get a ultra smooth wax coat on a sealing layer [ paint ] if the material is porous

a good book on fiberglass boat building is a good starting point

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Question wonder and be wierd
are you kind?

quote Originally posted by Curlrup: ...... West Marine makes a great epoxy resin system.

Great product.