[RustyLugNut]

I share the concerns others have voiced.

It may be only a few hundred USD to properly create a stressed glass reinforced fiber (GRF) skin in comparison to a Titebond (aliphatic resin ) glue shell. A proper GRF shell will have a minimal amount of resin. Most of your costs should be in your fiber cloths.

I had the opportunity to run into pretty much the same choice you are making right now. It was over 2 decades ago when I was part of a team prototyping a medical imaging system that had a large structural GRF shell. The glass over foam is commonly used for structural prototypes. However, in this instance, the foam core came to us in high density styrene instead of the usual urethane RenFoam. Because of the tight deadlines ( we often work on drop dead orders ) obtaining sufficient epoxy wasn't an option. We had to use the polyester resins we had on hand. A quick call to the local Ace Hardware and we were on the phone with someone from Titebond who gave us a quick rundown on how to build a protective shell of their glue ( original formula ) along with some "gossamer" cloth of some undefined makeup that is used in decorative sets. A light sanding once it had dried in the "hot room" and we were happily glassing away with the polyester we normally used. Metal backing plates and compression bolt tubes allowed us to hang the heavy radiation source and motors along with the patient bed and controls. The prototype was in operation for almost a year with no shell structure failures.

I do not know what the properties are for Titebond II and III but I assume it is much the same for the Original formula. The representative reminded us that the aliphatic resin glue has a creep strength ( cold flow or plastic flow under long periods of stress ) that are far below its ultimate tensile strength and yield stress. If the structure is to be under constant stress, he advised a safety factor of between 4-10 to take into account this creep. Temperature, humidity as well as the level of constant stress determine your level of safety factor.

I urge you to, at the least, consider the aliphatic resin glue as nothing more than a protective shell over your styrene foam core, then finish off with the normal polyester or epoxy if you feel you have to use it ( the PVA glue ). I would not worry about the use of the glue in the layer buildup of the foam as the core carries less stress flux than the shell. But, using the resin as a main part of your structure would mean a much thicker cross section to prevent the stress creep. You may inevitably lose your lightness and economy of construction.