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3177 Views 7 Replies 5 Participants Last post by  Dyer
I'm just not clear on what causes warping. I thought it was mostly about heat at first. I did some testing. I checked a piece of sheet metal with a laser themometer, it was at about 70 f. I hit it at 80 psi with walnut and it went up to 73 f . I didn't see any warping. After it cooled back to 70 I blasted with a fine glass at around 60 psi and it warped but it was still just around 73 f. I used glass again at 25 psi and I don't think it bent that time.

So is it a combination of the abrasion and the pressure it is being hit with? Is it weakened by the abrasion and forced out of shape by the air? It seems very hard to gaurentee no warpage. It takes a long time to remove rust at 25 psi. I will have to buy some small sheets at Lowes and test again while tracking the media and pressure each time.
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i was always under the impression that what warps metal was the actual size of grain of media like little tiny hammers or large hammers all work hardening the metal..or peening it.if you want almost zero warpagew you have to be pretty quick and move around a bunch.also if the paint job you are stripping is real thick like 5 coats thick then i would suggest soda then and gets real costly that way but hey its there project not yours...
That's what I figure if it has paint. The walnut first then glass.
If it is the peening doing the damage, however, then moving around in short bersts won't help. And you have to be on it enough to remove the rust eventually. I will be testing this carfully and I feel I will get my answer. I hope it isn't just luck based on the thickness of each part. Some customers may have seen some warping and just never mentioned it. Then you are fooled in to thinking it worked. No more guessing at this. I will test till I know and keep samples on my wall in the shop.
Here is what I found so far. I used a few five by three inch plates, a buck each that lowe's had for nailing wood together. I checked its temp first and then blasted it with med glass at 60 psi. It warped quite abit and its temp had gone up about ten degrees to 74. I then used the same med glass on the next one but at just under 30 psi and for a little less time. I did not see any warping and its temp had not gone up at all.

So it isn't just that soda or walnut don't build up heat. It is that they have not distressed the surface. When we scar the surface it weakens it and alows it to expand easily with just a little heat. I think if I blast a plate with out warping it and then hit it with walnut it will warp. Notice how a plate will warp towards you in the direction of the weakened side. I think the medium may work better than the fine glass. It may do rust removal more quickly so that you can get somthing done with only 30 psi or less. If you have to hang out with the fine longer to get anything done it may build up more heat. You can get a laser thermometer pretty cheap from harbor freight. It's a good tool to have for this kind of thing.
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we have done a lot of cars using medium crush glass at about 65psi and have had no problems with warping.
It must work if you keep moving enough I guess. As a long as we keep the heat down

Betterblast, The steel you are testing on is not the same grade as car steel. I have been out of town running all over the tri-states trying to wrap things up for the year. So I have not been able to post. I will not remove all the little black pitted spots on sensitive metal. You have to realize that the auto bodies and rod shops have converters and sanders for that work. Dont try to be too much of a perfectionist. Blast it, do the best job you are comfortable with and call it good. I have the guys who ***** come in the boothe. I let them tell me how much more they want Blasted so I am not liable for warping. This is a hard and dirty job, if you want to know how much time we save these guys buy a dual action sander and a roll of eighty grit sand paper. Take an old painted up fender and time yourself see how long it takes you to get that clean blasted look.
I have always been lead to believe (and it may be wrong), but heat will warp metal but the other factor is surface tension of the steel. When the steel is molded it is stretched to it form, when the surface is broken via sandblasting, grinding, welding or even rusted out spots it brakes the surface tension of steel.
The thinner the metal the more noticeable it is.
At the ship yard they would put heat on in spots then cool with water, sometimes there would be 5 dozen spots in one area, but they could pop out a warped area up to 1/2 ” steel.
The guys that would do this type of work tried to explain the surface tension to me?? I hope I got some of it right.
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