Starting on Stringers Someone may ask or comment about the foam, so I'll try to answer ahead. The most common foam boards are polystyrene or polyurethane. The problem with these foams is that fiberglass resin will cause them to melt, so they don't serve very well as molds for fiberglass. Unless they are painted or coated in some way. The test for whether a foam will stand up to fiberglass resin is if it will dissolve in acetone. Acetone is a very aggressive solvent that will melt many different kinds of plastic. Find a foam that won't dissolve in acetone, and it will work for fiberglass molding. The foam I am using is polycyanurate. It doesn't dissolve in acetone. Notice this foam is covered with aluminum foil. It's used most often for an insulating layer under home siding. Most guys who use this will peel the foil off. But really, I don't care how well the fiberglass adheres to the foam, because the foam won't contribute and real strength to the stringers. It's just a mold, and later will add some flotation. This foam is 3/4" thick, the same thickness as "1 by" lumber, and half the thickness of "2 by" lumber. In my mind, this is handy for making the long stringers, which must be spliced together. The stringers are longer than the foam sheets. You'll see this in the next post. To glue the pieces together, I'm using 3M "Super 77" contact cement. This is their heaviest-duty contact cement, used in cars for sticking up headliners.
