The sun has damaged the plastic resins in the original layup. The roof you are describing must be a single layer, and you can rebuild the thickness of the fiberglass by adding more materials.

If you added a layer of 1 1/2 oz CSM  (chopped strand mat) it would increase the thickness and rigidity of the original roof. It can be sanded smooth after it dries and painted over. A better roof can be built using multiple layers of alternating 3/4 oz CSM and 4 oz cloth.

If you're just trying to fix it for resale instead of keeping the trailer, it may not be worth the extra effort past a single application.

Gard

I use both of these suppliers. Mertons has a better supply of different resins, both air inhibited and waxed. You can also obtain resins in gal and 5 gal amounts without a special order. Both have fast shipping and excellent service.

http://www.mertons.com/

http://www.jamestowndistributors.com/userportal/search_category.do?categoryName=Fiberglass%20Supplies&categoryId=373&page=GRID

Let me know what you want to do and we can discuss a more detailed plan of repair.

Gard