I did some testing on adhesion last winter and found rough sanding the foam was not the best solution for adhesion at least not with the materials that I used. Sanding may work fine on urathane foam, I don't know.
I should also say the test was only a peel test and I used the exact materials that I was using on my build. Foamular C-300 from Home Depot / U.S. Composites 3 to 1 thin epoxy 635 / 6 oz e glass fiberglass. I tested various grits of sandpaper from 220 down to 20 grit (had to glue the 20 grit to a block of wood and kind of twist it into the foam) as well as drilled holes and holes poked with a scratch awl also with and without 3m super 77 spray glue.
The only sample I saved was the one I found to give excellent results which was scoring the foam not rough sanding. I scored the surface of the foam with a stainless steel pet brush from the dollar store. This only seems to work well if you score lenghwise and what you are looking for is score lines like in the pic . If you use a plastic squeege to spead the epoxy it seems to "key" into the foam quite nicely and gives a really strong bond. So strong you can rip the cured skin of fiberglass (6oz.) before it will detach from the foam.
If you don't have a dog or cat then a gang of utility knife blades with some thin wood spacers between them works at least as well as the brush.
I realize that there is lots of other methods as well, like holes drilled or cut and wood dowels or blocks of wood inserted to join the two skins but the scoring method is nice if you have a large area to do like I did.
Just thought this might be of interest to someone since this bond is so critical to success.
