Thought I said how we filled in a 2 x 3 foot holein the side of our trailer: we got a flat piece of fiberglass (cut out of a bathroom wall) and cut it smaller than the hole. We taped it in and began fiberglassing around it layer by layer. As it became more solid, we took the tape off and filled in those gaps as well.
We faired it on the outside, and re-fiberglassed it again where it was really too low. Again, faired (using Bondo Hair product, a fiberglass putty kind of stuff) and kept fairing and sanding until it was absolutely smooth. Then we washed it with dish detergent and soap, then wiped it down with acetone. Then we carefully and thinly primered it with Rustoleum Marine Paint
(white) and then again, sanded with super fine sandpaper (1000 grit). Wiped with a tack cloth. Then put two coats of white Rustoleum Marine enamel over it, using a small, hot-dog foam roller, rolling until all bubbles were popped. Sanded between coats. We were painting
the whole trailer anyway, with this paint
and primer, so it blended in very well, being good at self-levelling plus Paul is nearly obsessive with detail. INSIDE the trailer, we again worked on it, filling the low spots, sanding, fairing until it was smooth.
If we hadn't had the big piece of extra fiberglass, we'd have BACKED the hole with a piece of thin plywood paneling (probably on the inside) holding it in place with tape and covered with waxed paper OR simply incorporating the ply into the patch, using NO waxed paper and working over the ply inside and out until it was flush on the outside and smooth on the inside. We'd have layered the fiberglass "onto" the waxed paper layer by layer, using mat cloth and epoxy resin. We'd have kept at it until it was flush on the outside, taking the tape off and working on any gaps afterwards.
In many places, the inside of our trailer (where there are large flat panels someone (maybe the manufacturer) fiberglassed large plywood panels over the shell. INSIDE. Then that panel becomes quite strong and even rigid.
Anyway, that's how we did it. There are MANY YouTube videos showing how to do fiberglassing, a few hours spent watching them is a good education in it.
We would INSTANTLY be willing to work with fiberglass to repair the shell of our trailer; it's easier than it seems, it goes well, if you make a mistake, just sand it or cut it away and start again. You can use the broken bits and fiberglass over them, around them, reusing them to save on materials...
It takes a lot of sanding and prep, it takes a lot of material and money for the products, but in the end, Paul was able to fill over 80 holes to the point where they are literally invisible. We feel the trailer is much stronger now, all the big cracks radiating from around the doorway are filled and solid, the sideways cracks along the body are filled, everywhere a metal item was removed (logos, vents, etc) are all filled and totally flush with the skin...and thicker inside for reinforcement. He fiberglassed in three areas of thin plywood inside for reinforcement, and it is super.
Use a mask when sanding...find out more about polyester resin online or from someone who's done it.
Epoxy resin is pricier, I hear, but it worked up very nicely and you can also look at the thread here on FGRV under SEARCH: Fear of Fiberglassing. The advice was priceless!