Thanks, Patricia D! I re-read what I wrote there, and have added a bit here. Our paint job so far is holding up well. A few rock chips on the front, but only rubber paint would have helped with that.
Bondo is OK but "Bondo HAIR" is for fiberglass. If you fill imperfections with plain Bondo you may find it popping out later on, even if you paint over it. So use Bondo HAIR. It has tiny strands of fiberglass in it and is stronger and more tenacious. If I had a fiberglass body CAR I'd use Bondo HAIR to fill dings and chips. We used Bondo Hair on Peanut and haven't lost a bit of it. Nice and solid.
Be sure to sand first; yes, you can use a coarser grit originally, and to clean out small dings and chips--you can use a little Dremmel and try to slightly undercut so the chip is bigger underneath than at the surface. That really helps your Bondo Hair stay in place.
We rolled ours with a foam hot dog roller--Paul rolled it thoroughly and got all the little bubbles popped with the roller--and then it self-levelled as it dried. It's nice paint. We didn't spray.
We took the windows
OUT first--that's the preferred way--but you can mask around them (and over them if you're spraying). Mask very carefully, meticulously.
For finish work, after you get the first primer coat on (and I agree and recommend marine
enamels--Brightside is one of the really good and recommended ones, but is pricier and less easily available than Rustoleum Marine enamels--then you use increasingly fine sandpaper so you still have a nearly microscopic "tooth" on the surface but an easier and easier time of getting the final surface smooth. Haven't used Sherwin Williams Marine Polyurethane yet, but it's no doubt a good paint, too. If you buy your marine paint by the quart, get 3-4 to start with and you can return what you haven't opened or keep it for touching up later. Rustoleum was about $23 a quart around here--I believe we used 2 quarts of primer and about the same of topside (top coat--finish--main paint).
After you use Bondo Hair or any fiberglass patching, you should wash the set-up and cooled area (fiberglass heats up as it cures) with plain soap and water (you can use dish detergent but not dishwasher detergent, though we never tried that), rinse very well, and then wipe it down well with acetate. You can get acetate at almost any hardware or auto parts store. It's also in fingernail polish remover--along with scents and color and emollients. You don't need those.
Use rags and throw away the dirty ones. You don't want any dirt or dust in this project. You already know that, though!
Let it dry very well before proceeding.
The soap helps neutralize the chemical surface of the Bondo or fiberglass (far better explanations are available) making it less likely to reject the paint. Otherwise you can get a "hard" or "hot" spot that will cause the paint to pull away from that area. You don't want that.
Ken's method isn't essentially different, and I agree with him about some practice time if you're not used to this sort of thing You might even try practicing with the Bondo Hair to fill some cracks/dings in something and then paint that. Paul (the ball and chain) practiced on our old lawnmower deck--which had rusted through. He did it, a little lumpy, painted it with leftover primer and red enamel and it's really quite good. He got better as he went along.
Wishing you the fun and satisfaction that comes with fixing up your trailer--
And then, HAPPY TRAILS.
I see this isn't such a "summary update" after all.
But it's a subject near and dear to my heart.