Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - my doors got stretched
View Single Post
  #11 (permalink)  
Old 11-26-2004, 09:14 AM
Brian Martin,Freelance adviser
Last photo:
Join Date: Jun 2004
Location: San francisco bay area
Age: 58
Posts: 15,492
Wiki Edits: 0

Thanks: 4,316
Thanked 3,732 Times in 2,765 Posts
Randy, this is a copy of an email I received after posting about sand blasting. I thought it sounded pretty reasonable, what do you think?

Sandblast warping Sandblast warping
XXXXX XXXX here - I read your post / recommendations against sandblasting body parts and I am in complete agreement. However, the mechanism that creates warpage is not the heat associated with friction of the abrasives hitting the metal. Rather it is the compressive forces induced within the sheet of metal by the heavy abrasive particles hitting the metal surface. You will not see this same warping when light abrasives are used because they lack the kinetic energy required to compress the molecules in the steel sheet thus leaving the sheet in the same stress state as it was in when formed by the dies at the factory.
This is a common misunderstanding, yet this behavior is well understood by some industries and is exploited by those who understand it to improve fatigue life of certain parts (like connecting rods for instance - they are often shot peened to redistribute stresses and place the surface of the rods in compression to reduce fatigue crack propagation).
When you blast one side of sheet metal, that side of the sheet metal surface will build compressive stresses and will physically grow - this places the opposite side of the sheet into tension - this will result in the side being blasted to bow up toward the blasting source.
In order for warping to occur due to thermal conditioning - the metal actually has to be heated up to a point where the molecules reach a transition state where they go from one packing arrangement to another (close cubic packed to close hex packed) - picture a six pack of beer held together with the plastic rings - that is stress relieved / annealed steel. Now - take the cans out and stack them so that each row of cans is offset by half a can diameter - this is a tighter pack - although less friendly from a packaging standpoint at the grocery store. This new denser orientation is the orientation that steel molecules take on when raised to a sufficient temperature (depends on carbon content and alloys) . . . if you raise steel to this temperature and then quench it - you lock the close packing orientation into this dense packing structure and this results in shrinkage (and increased hardness / brittleness).
Using this technique it is possible to take a piece of 1013 mild steel, whack a piece off, heat the piece up and then drop it in a bucket of water, use a water cooled grinder to sharpen it . . . and then you can cut the parent piece of metal with it as it is harder and denser in this condition.
Soooooo . . . . that is the difference between warping / shrinkage of metal due to sandblasting (compressive mechanism - localized molecular packing) or due to thermal influences (gross - macroscopic transition of molecules to a higher density due to re-orientation of molecular packing).
With all this knowledge in hand (or mind) . . . you now also know that thermal conditioning is typically used to shrink metal on a localized basis where sandblasting will always cause metal to grow on a localized basis.
Hope this makes sense . . . thanks for the great writeup!

Jessie, I can attest this damage is not limited to late model cars. I understand it was a large flat panel like a late model, but the "good old steel" will do it just the same. I had a 1928 Buick door and a pair of 32 Ford doors RUINED by a sandblaster. I KNEW not to sandblast these parts to "white steel" but I figured a "dusting" wouldn't hurt. I was doing them for a display at a convention (ala NACE) for NAPA auto parts. The Buick door was SUPER straight and I was just going to have it lightly blasted, I would apply a coat of polyester primer, sand it and paint it. Then have a NAPA logo from the 1930s put on it like it was from an old delivery truck, cute idea huh?

So I took it to a sandblaster that I have had do a lot of work for me and my brother over the years. He did the 1922 Buick Roadster body on my brothers car, MANY frames, inner fenders, etc. over the years, he "seemed" to know what he was doing.

He barely blasted this thing, 90% of the surface rust was still there, he didn't bare down on it at all.
The door was TOTALLY RUINED (so were the duece doors but they were junk to begin with) I REALLY wanted to do this display and it was the only door I had so I had to "fix" it. I shrank it with a torch and a ROSE BUD tip! It was sooooooo screwed up it took me hours of work to "fix". I say "fix" because after I got it so it wouldn't oilcan I went to my "stock" of filler and mixed up a gallon for it! It ended up being DAMN straight, looked pretty good actually. It got a good coat of polyester primer, sanded and shot with an SS urethane bright red and the NAPA logo. It still hangs in the office of one of my favorite customers. It weighed a lot more than original, hope it doesn't fall on someone.

Last edited by MARTINSR; 11-26-2004 at 11:03 AM.
Reply With Quote