Most of the damage you'll ever see from media blasting will come from too much pressure or too coarse of a media being used. When each and every particle of media hits the panel, it's like a tiny hammer head hitting the panel. Too coarse of media is like using a larger hammer, too much pressure is like swinging the hammer harder. All these strikes against the panel causes a peening action on the panel but only on the side that is being blasted. So when excessive pressure is applied this peening action on the one side of the panel causes that side to stretch, and any damage done therefore will stretch towards the source of the media stream. The peening action occurs when the surface tension of the sheet metal has been exceeded. Soda blasting does not have sufficient "hammer size" to upset the surface tension of the panel and will only remove paint. Sand, garnet, coal slag, all have plenty of heft to upset the balance. I use this only as an example and not a recommendation, media will be covered later. Next, we get into the infamous old wives tale, sand blasting causes heat and the heat caused the stretch. Actually, the peening action of the millions of "hammer hits" against the panel is what caused the stretch. Take a piece of sheet metal and place it on an anvil. With only a few strikes of a hammer, you can see deformity as the panel will have stretched. The panel will still feel cold to the touch. Keep striking the same area repeatedly and soon you will note an increase in temperature. Just as in any other instance of a peening action from hammer strikes, no matter what the hammer size, the peening action causes the stretch, any temperature change is a side effect of excessive peening. Stretch has long occurred before any appreciable temperature change has taken place.
Media types
Soda blasting, as mentioned above, only removes the paint. Where this might seem the ideal media to use, it is not without side effects. The soda will also change the PH level on the surface of the panel to more alkaline, which is not conducive to paint adhesion. Epoxy primer sprayed over the top of freshly soda blasted metal will peel off in sheets once it has dried. Soda blasted metal, similar to using acid, must be neutralized prior to spraying with epoxy primer. any and all residue, such as any that would collect in the bottom of floor, rocker panel, etc. would need to be totally removed to ensure optimal paint adhesion.
Sand (many times packaged as "Play sand") in addition to the release of silica dust particles as the grains break down (Group 1 carcinogen), also contains caustic salts. Now imagine using air pressure to impale tiny particles of salt into your panel. I've seen on many occasion that someone's long term project had a spot that was sanded through as they got to the blocking stage of their paint job. Many had what they described as tiny specs of rust UNDER the paint. In all of those cases, SAND blasting had been used to prep the metal and the car had been sitting for over a year (inside storage). Therefore, blasting with sand as a media has it's side effects as well, and some form of pre-clean should be used to remove any and all traces of salts prior to painting. Another downside to pre-cleaning is that any rag used will undoubtedly leave behind lint particles in the now roughened surface of the panel.
Garnet and coal slag I'll group together, these are basically a media that is more inert in that it doesn't leave behind salts or change the PH of a panel. Both are definitely in the large enough hammer to upset surface tension category, so the operator needs to use minimal air pressure and smallest sized media possible where paint is removed but sheet metal is not warped. I'd suggest the least amount of pressure that will still remove paint. Either of these are my go-to media, and are readily available at places like Tractor supply or most industrial sales type businesses.
Dustless blasting, this one plays on the hype that heat causes warping and this process can't warp because we keep the panel cool. Yeah, right. Given enough media size and/or pressure (hammer swing) anything that upsets the surface tension is going to peen the panel and cause stretching. No matter what is pushing the swing. Now let's talk about cleanup. We've all seen how FUN it is to remove sandblast media, using an air hose for the better part of a day to get in all the nooks and crannies to make sure all the media has been removed. Now add water to the mix. This slurry is not going to give up easily, and any residue left behind is now your next water collection point/new rust generator. If someone were to use it in cleaning a panel separate from the vehicle that had no nooks and crannies for the slurry to collect inside, this may be a viable option. For anywhere that does indeed have the nooks and crannies, it's near impossible to remove all the residue.
Damage areas
The most prone areas I have seen in producing media blast damage are metal structures with openings, such as inner door, hood or trunk inner bracing, roof skin bracing. Most of these are littered with access holes and bead/bend details that makes them difficult at best to used typical sanding devices. Media doesn't care about surface irregularities, which makes in an ideal medium for paint removal. The downside here is that someone is so focused on removing paint from the hood brace that they completely miss the fact that we're also hitting the back side of the hood skin adjacent to the brace, coupled with ZERO media hitting the skin underneath the brace. This damage is readily seen as a low from the outside of the panel and, what a mystery, looks exactly like the structure's hole openings. They are always a low when looking at the outside as the peening action from underneath causes the stretch on the underside of the panel, causing it to gravitate towards the stream of media.
My recommendation for media blasting: light pressure and small sized (80 grit or finer) are key in keeping any damage at bay, I prefer garnet or coal slag media as I try to eliminate adhesion side effects in the products used. For issues when blasting inner structures, slip some cardboard behind or around the edges to block any media from slipping past the openings and getting to the outer skin. If possible, direct your media stream at an angle to help remove those direct hammer hits when blasting any large flat areas. Just like a glancing blow with a hammer inflicts less damage than a direct hit, an angled approach with the media stream will also lessen the blow and help to prevent warping. For doing areas like windshield/window flanges, they have sufficient structural support that they are less likely to be damaged by any peening action. So you could also address all the structural areas that are harder to abrade otherwise (door jambs, window openings, etc) and once complete, the large flat areas, if needed, could be addressed with less intrusive methods, like a DA sander.
Once all the media blasting and abrading is complete, then I'll spray epoxy to keep the panels from having any flash rust issues. Keep in mind that for spot repairs as you work through the various areas needing bodywork, any bare spots uncovered can be addressed after bodywork by spraying, rolling, or brushing on more mixed epoxy to keep any flash rust at bay. You'll need to block out afterwards regardless, so whats a few brush strokes going to hurt?
