My leer truck cap on my Dodge that I installed a fully welded steel ladder rack on top is stress cracking.  I work construction and tree farming so the rack was over built and overloaded probably more than once.

Now that you know what I did wrong I'll tell you what I want to do...  I found a quart of Toyota army green leftover from jamming out the truck before sending it to paint, so I figured might as well fix the topper and finally paint it to match the truck.  Last weekend I sanded most of the old paint off one side was done with 150 one side was 60(I'll explain in a moment.)  Now that the paint is stripped I'm seeing pretty deep stress cracks, and the fiberglass topper has a complete 1/8 inch skim coat of light weight filler.

Now explaing the sanding one side was done with 150 to investigate issues once the problem was found.  I talked with my dad and a couple of car guys and they all said sand it with 60 re resin the topper and smooth out with filler or grind off all the filler and repair the fiberglass that is cracked then rebuild filler.  

I plan to keep the rack so inside the topper we will be classing in metal arches to take the load down to the bed rails.  My concern is will I have massive amounts of block sanding if I sand to 60 a re resin it seems really rough.

Here are some more pictures I had someone suggest just using Kevlar reinforced filler and just filling in the cracks the side that got hit with 60 almost removed all the small cracks but the big ones on all 4 corners weren't affected

The glass and resin is the hardest to sand.  We use 16 or 24 grit on that. Gelcoat isn't easy but better than glass

 

We use a special filler that sands pretty easy.  We also use a supercharged type of bondo that ain't too bad. Our favorite is lots of primer. Spray as much primer as you can even if it takes multiple sprays. It's the best for fairing.

 

Good luck, have fun & I hope you feel better.

 

L8tr

D