rruff wrote:IME PL300 isn't nearly as strong. I have no idea why the manufacturer advises against PLP for foam, since I've never heard of anyone having an issue with it.
My guess is that it has to do with the typically intended application, hanging foam board on vertical gypsum or concrete walls prior to having another wall board material fastened down over it. The foam will swell and contract at a different thermal rate and the rubbery consistency of the PL300 will allow for more give, especially when applied as per the MFG's instructions (i.e. thick vertical beads placed 12 inches apart). My interpretation of the instructions is that they really only intend for the PL300 to "baste stitch" the foam board in place, that the spacing and thickness of the recommended beads allows for a chimney affect that allows the adhesive to cure, and that the long term fastening is by what ever mechanical method is used to secure the overlay.
PLP, on the other hand, cures much harder and may not result in a very flexible bond.
Either method troweled or spread on will likely run into curing issues due to creating and airtight seal around the perimeter before the center has cured. (I used kerf chimneys in my foam to combat this.)