I ran across this brief piece on Ecomodder.com where the author did adhesion tests to XPS.   I've played with this aspect of foam and glass, and not come up with a good solution myself... sanding & sand blasting the surface lightly were my approaches.   This author used a pet brush and scoured the surface fairly significantly..... as one of his tests, and this yielded the best results.  The result is more or less like knife cutting, in fact the author suggested a gang of utility knife blades as an alternative.  

     I believe that there are many places where blue or pink XPS insulation foam core would be appropriate on a boat.  It offers both a much greater R value (insulation), and considerably lower cost.  It is lighter weight / lower density, and not available less than one inch thickness, though StruCell should bring foam core construction down to the "reasonable" price range for most of us.   I eagerly await results on that one!!

https://ecomodder.com/forum/showthread.php/improve-bond-fiberglass-epoxy-home-depot-foam-extruded-24820.html

                                                                                                          H.W.