[dsraven]

I have done my own sandblasting on several automotive projects. the trick is to keep moving and stay back from the surface. hit the surface at an angle not straight on. don't concentrate on a particular spot till it is bare and clean, move the blast pattern around the project and just keep hitting that spot time and again till it's good. also vary the angle so you don't always hit part "A" at a 30 deg angle from the same side, this way you get all the "shadow" areas. sandblasting also leaves a profile on the surface of the steel, like the surface of a piece of sandpaper with highs and lows. if you don't sand it before epoxy then the tops of those craters can stick up through the epoxy layer, like a mountain top through the clouds, and become the bare steel that turns to rust. I recommend to hammer out some of the larger dents before you blast, even use a disc grinder with the sand disc to remove known filler areas, then blast, then sand with something like 180 grit to knock off the mountain peaks, then wipe it down with wax and grease remover, then epoxy right away to seal it. let the epoxy cure well and then do your hammer and dolly to get the shape back before any welding which will also try to reshape the project part. it may even be a good idea to fit the part where it will be to ensure it still fits before going further with weld repairs etc especially for fenders and stuff that can be mishapen but looks right until you hold it up where it fits. better to get it shaped right before it has a chance to warp I think. then after welding and hammer/dolly/metalworking re-epoxy before filler. because filler is porous it can allow humidity to hold under it and cause rust. I always try to ensure the filler goes on top of epoxy. primer surfacers are also porous so it is a good to know your product before you spray so you have an idea of what to expect. if you do the bodywork and expect to have the project sitting arond for a bit of time before paint it could be a good idea to re-epoxy after the bodywork is done so it is sealed. thin the epoxy so it latys out smooth and will take only a bit of work to sand it before sealer and paint.
I know guys who spent time and money on sandblasting stuff, like machinery or trailers, and then sprayed primer on the parts friday night before heading out, thinking it was sealed. well, primer isn't always a sealer so the stuff was rusty when they came back after a weekend off.
talk to your blaster to ensure they have done bodywork with success. see some pics or actual stuff they have done on sheet metal if possible. they may even offer to epoxy the parts for a fee but then you don't get to see how good the blasting was. anyway, blast, sand, epoxy seal, thats how I do it.
good luck, post up some before and after pics.