I started doing some bodywork on my 87, I dont have a welder, so I am grinding and wire brushing the rust, then I etch the metal, apply por 15, and paint, it is working okay, some places are bad so I have some good size holes, if I had a welder I would weld in patches before the painting, using what I have, how could I fill those holes? One option is to patch with sheet metal and rivets then spackle and paint, seems a bit lame, hmm...
here is a few pics

 

Instead of leaving the multiple small holes, you could just cut out an entire length and then weld on a new sheet. It would definitely look cleaner than the rivet idea. The same would go for other panels/bed rust spots.

 

I am in about the same situation with my truck. Right now I dont have a welder. If I dont have one when I get to that stage, I will do the same thing as you and then put small sheets of fiberglass on back and fill it with fiber glass resin.

 

I would do that if I had a welder, I am off grid so a I would need to buy a gas powered welder, thats more investment than I want to pursue, I agree that is the best way.

 

Interesting, will fiberglass bond to the paint allright?
Thank you both for the feedback

 

Here is the truck before and now, I took off the rusty step up bars that were infecting the frame, and I took out the rusty roll cage, also took off the cheesy flares, its starting to look cool.

 

Here are the pics.