so, in the process of roof rebuild on 1985 Coleman sun valley pup. Reading all the forums I’ve seen many discussions of adhesive failures. Titebond, liquid nails, spray adhesives, what have you. First off, the plywood on the corners. I used phenoseal which is working great; I had it on hand and know it to be sticky. Lexel would probably do as well, if not better. The trick is something that will stick to the aluminum, plywood is porous and easy to stick to for almost any adhesive. So, since phenoseal will stick to the aluminum, it was an easy choice. Any type of gutter seal would probably do as well. Now there’s the whole gluing new foam to the aluminum.this is where the forums fell short. Yes, there are adhesives made specifically for foamboard. Yes, they will glue Luan to foam if you are planning to use Luan for you finished ceiling. Here’s the trick of it, luan is porous, air passes through it. These adhesives, and most any caulk, need air to cure. Hence the foamboard pealing away or the frp pealing away, weeks after it was installed only to find uncured glue, wet and sticky like it just came out of the tube. What we need is an anaerobic adhesive that will bond to both aluminum and foam.. And then foam to frp. It exists, I can’t find it. Or if I did, I found it prohibitively expensive. So, a trick. I tried gorilla glue as it claimed it sticks to metal... it did not. But it stuck to phenoseal. So, I growled out phenoseal on the aluminum with a notched trowel, like you would use for tile of vinyl flooring. I allowed that to cure for a few days. I then smeared out a thin layer of gorilla glue on the foam, which it will stick to, spritzed it with water, as that, not air, is what makes it cure, and clamped the foam to the aluminum. 1-2 hours set time, it foams up and grabs the phenoseal layer, 24 hour total cure time and bingo, foam glued to aluminum. No uncured mess hiding between the layers. Will probably end up doing similar with frp to foam. The main point is, when dealing with nonporous materials, you need glue which can cure without air exposure.