Join Date: Jan 2013
Location: Boise, Idaho
Bronco Info: 1988 XLT 5.0/AOD
Never leave body filler exposed to elements. If it's in a garage or shop, then it'll be ok for an overnight thing, but if it's left any longer then you'll experience cracking down the road.
As stated, use body filler and sand in the following steps of paper: 40 grit, 80 grit, then finish it off with 180. Then use some evercoat over the top of the body filler to fill any pin holes and scratches leftover from the 40/80 grit. Also, NEVER use a DA or sander on body filler. It won't be level. Use a long board with sand paper and do criss cross sanding in all directions until it's straight.
After sanding and getting it smooth, you can use a primer filler to fill in any scratch marks or pin holes in the filler as well. Primer filler can be via spray gun or rattle can, just be sure to get the good stuff if you do a rattle can. Paint supply shops sell it, it's just as expensive as a gallon of primer though. For primer, you can get cheap priming guns at paint shops for about $80 or even harbor freight sells them for $40-60. Doesn't need to be anything special, just make sure the tip is 1.4-1.6 millimeters.
As Fox said as well, after you're done with the filler and before primer, spray it was a very thin coat of black spray paint or pick up the black powder from the paint supply shop. With a long board and 400 grit, sand in criss cross and find the leftover black. Fill it, and repeat.
Do not use cardboard for body filler mixing, the cardboard can lead to the same break down of body filler as leaving it exposed to elements. Cracks in the filler will follow at a later time. Use an old glass window, or a plastic/metal board. Paint supply shops sell mixing boards for like $3.
When using filler, you shouldn't need more than a 1/4" of filler over dents. If you need it thicker than that, you need to do more body work to get it straighter. Anything over 1/4" thick will....again, crack.
A DA (air sander) can be used to sand the rest of the body. Be sure not to use the sander on body lines though, only use a block on it. DA's can wear down body lines and the finished outcome after paint will be disappointing. 320 grit on a DA will take down just about everything you need it to.
As for primer after all the body work is done, I'd finish it off with a primer sealer. Spray down a regular primer, wet sand the entire vehicle with 400 grit, 600 grit, then finish it off with 800 grit. After sanding is complete, spray your primer sealer and leave it as is. You can leave it exposed for a few days outside with this before spraying your paint. Don't leave it out too long exposed though, there's no UV protection and although the primer may appear glossy, it can still break down and cause the paint to bubble a few months-years down the road. When you're ready for paint, wet sand again with 800, then 1000, then 1500. After this is complete, spray your paint and clear (or single stage paint) and you'll be good to go.
Most importantly, take your time. If you want the truck to look good, you're going to have to make sure everything on it is straight. Darker colors will show your body imperfections, so just be aware that the amount of work you put into your paint job will reflect on the outcome.
88 Bronco XLT - 88 Suburban 2500 - 01 Ram 1500 - 01 Suburban 1500 - 14 Expedition - 08 Sierra 2500