Hi All,
I haven’t seen a lot of discussion about glue types for XPS when gluing sheets together. I did see that some people had success with titebond III (the waterproof one), and mixed feelings on gorilla glue because the PU in it may melt the foam and/or expand. I see that great stuff works well, and epoxy or epoxy w/ microballoons too. I can say that I found out this week that using an air drying glue(titebond) with XPS is a huge mistake - unlike EPS, XPS is totally closed cell, there is no air exposure at all, it never dries. NOTE: this only applies to foam to foam joints. I imagine a foam to wood stringer joint would still cure properly.
Background: I built a rocker table and was gluing up 2 sheets of 2" GreenGuard/Pactiv 2# XPS (green). Spread a thin layer of Titebond III on both surfaces and pressed on the rocker table. 24 hours later removed weights and both sheets delaminated, the glue was still wet everywhere except the very edges. Shrugged, added weights back on, drilled a couple hundred very small holes through foam for circulation (I was contemplating the holes per surfer steve anti-delam recommendation anyway) and let it sit for a week on the rocker table. Took the weights off today and was hopeful at first, but the sheets delaminated as soon as I flexed the block.
Lesson learned the hard way.
I ordered 2 gallons of epoxy for glassing last week, should be here tomorrow. I also thought about using GreatStuff, but without a stringer, I figure the epoxy should help keep it from being too floppy until I can get some glass on it. Unless someone has a better suggestion, I’ll be using epoxy and will try to keep it about 1" from the outline for easy shaping.
This did get me thinking though about the delam issues with xps. If great stuff forms such a good bond to XPS, what about putting a skim coat of great stuff on a shaped board, sanding out the bumps, to leave a thin fuzzy layer of polyurethane, then glassing? Do you all think the epoxy would get a better bond? Better shear strength when dented? I might try it with some scraps. Even if it does work well, it might still be difficult/impossible to do evenly on a whole board.