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MG MGA - Sandblasting panels

I believe that sandblasting steel panels can, under certain conditions, cause warping. Is this because of (1) heat generated at the metal surface (2) metal stress induced or relieved or (3) the force of the particles hitting the surface? What are the conditions necessary i.e. air pressure/particle type/particle size to avoid warping?
Some time ago I took a gearbox tunnel for blasting and the firm refused to do it saying that they used 'chilled iron' at around 100 psi and it could warp.
Another outfit advised if recycled chilled iron were used warping would be avoided. There was no mention of grit size or air pressure/volume. In the end I decided not to risk it and used a power sander instead. However, I don't want to use this method for wings and body but I want to be sure that conditions used are risk free.
m.j. moore

I just had all the wings beadblasted, they came out looking great. I would check with the person doing the work, then try to check with some of their customers.
Blast Teck here in colorado work on alot of classic cars.
Good Luck!
D.M. Black

When I took my fenders down to the body shop for a pre-body-shop-job-inspection, I asked if I should have them sandblasted, and the owner told me that sand-blasting could cause warping.. I took them to the sand blast shop and asked the guy that runs it about the warping. His answer was that, yes, sometimes it could. He explained that it was a combination of heat, impact due to the 150 pounds of high volume air pressure, and some other things, so he blasted them with little plastic foamy pellet things- did a nice job and no problems what so ever ( except that they get jammed in a lot of little crevases )----sure was a lot more economical than paying the body shop to hand sand them!!! I also did the same thing with the bonnet and the boot that I was concerned about- being aluminum- but they also came out fine..
FWIW-- / gil
gil

I had always thought it was the heat generated that caused the warping but was chastised on another forum that it is in fact the force of the particles hitting the panel. Whatever the case may be, media blasting is by far the only method i would ever use on a panel. Soda, plastic pellets or what have you........anything but "sand". Good advice given here, check with the person doing the work, and talk to some previous customers...............
Mitch Smith

The trick is, you've got to find a guy who does cars as a main part of his business. If you find a guy who says that blasting could warp the panels, then he probably will warp the panels. His skills and equipment may be more for removing graffiti and paint from bridges. Search around through local car circles, and you'll find somebody with the right skills. And they like to call it media blasting these days, at least in the US. Nobody actually uses sand anymore.
Steve Trovato

I did an extensive bit of research on paint removal on aircraft for an airline I worked for. The results of what I found out were that sandblasting should be used only for frames and heavy structures and even on those there can be a considerable amount of good material removed. The Plastic Media that was used by the airlines for a while is good on body panels only if used by very well trained personel. It will warp panels due to peening out of the surface and heat generated, I would highly recommend against using it on the doors, hood and deck lid. The aluminum skin on them is much softer and more malleable than aircraft skin and would be very easily warped by blasting with anything but a Carbon Dioxide blaster which is very expensive to operate and uncommon. The aluminum skinned parts would be much safer to strip with an aircraft grade of chemical stripper and some hand work. Be sure to use the proper safety equipment though. I also looked into an Electrolytic method of rust removal when I did my "A" that looked really good, a little expensive but if I had had much rust I would have used it for the steel body components. Parts come out of that method completely stripped with absolutely no rust remaining even in the bottom of the pits. Hope this helps.
John Nelson
J E NELSON

The effect of blasting can be controlled quite easily by the distance of the gun from the work. For best results on a sturdy workpiece, I go for about 1 inch away. But if you back off to 2-3 inches there is much less effect, but it will take paint off. If you adjust the distance so as to remove only the paint it will not warp. Doesn't matter what media if you use this principle.
Art

The angle at which the panel is attacked also determines the amount of heat and distortion caused. A direct blast is most problematic, while a 45 degree angle strips quickly with less distortion. The greater the angle, the less distortion, but the slower the stripping process.
Jeff Schlemmer

I'm sure you're right Jeff, but my point was, whatever media / angle / air pressure used, if it is just enough to strip paint, it can't damage the metal. So back off on distance and approach unril the paint only is stripped.
Art

This thread was discussed between 13/05/2006 and 15/05/2006

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