The Sikaflex rep that I spoke to was extremely helpful and advised me that the fibreglass should be prepped by giving it a rough sanding and then clean with warm water and a little bit of dish soap (non-scented to avoid oils). I did that and went as far as to spray the interior to rinse it off well. That led to an unhappy discovery....more on than later. The rep also said this adhesive works best leaving it as a bead at least 1/8th" thick (i.e. don't press the strips tight to the hull). I think one of the things I most enjoy in this project (and any project), is the problem solving that is required! In order to ensure that I left a 1/8" thick bead, I cut 3/8" wide spacers from 1/8" thick baltic birch plywood that I attached to the strips of bending ply spaced approximately 3" apart. I made the spacers slightly less long than the width of the plywood strips (1" or 1 1/2"). I had elaborate plans of making adjustable length braces to hold the first layer of strips in place. On further thought, this seemed slow and cumbersome! How to get the 1/8" spacers to stick to fibreglass...
I tried double sided tape....not strong enough. What about hot melt glue?! More than strong enough (in fact it sticks really well!!) but with a very short open time, i was limited to gluing 12" long strips. That was ok! The very repetitious process began...
1) Prep 12" long strips of bending ply by nailing spacers 3" apart with 3/8" 23 gauge pin nails (my poor pin nailer owes me no favours now
)
2) Apply beads of Sikaflex 252 between the spacers (I didn't skimp on adhesive). I discovered that this adhesive is so thick that a special caulking gun was required. Even with that it was very hard work to use a manual gun. No way I could do the whole trailer with a manual gun! My new best friend...a Ryobi cordless caulking gun!! Set on a very slow speed it worked awesomely!!
3) Apply hot glue to the 4 spacers per strip and very quickly press it into place pressing the spacers tight to the hull. The strips adhered very well and followed the curves of the trailer beautifully
4) Repeat over and over and over....