stripped to the bone

[Don Jordan]

I'm still new at this picture thing - I hope this comes out. I just went down to see my car. Next week it's going to the sand blaster. It's all kind of scary to me. I just know that I never could have done it.

[Normspeed]

I'll bet when it all comes back from the blaster, you'll be smilin'. How long will they have it?

[PatS....]

I hope they know what they are doing. Sand can destroy sheet metal in big hurry and should never be used on sheet metal.

Glass, walnut shells and such are much safer but still require caution.

Good luck.

[Normspeed]

I've wondered about that Pat. A friend of mine recently completely sand blasted a 50 Ford panel truck using sand and he's happy with the results. He's reassembling it as we speak.

[Plymouthy Adams]

Sandblasting is not so bad if you are cautious and do not over pressure the media..the high pressure and extreme sandflow will heat the metal and cause warping to no end..the "glow" can be seen if pressure and flow is high..

If they use moderate pressure and sandflow, correct distance and keep the media moving over the surace, there shoud be no damage..I do hope that they have covered areas like suspenison and engine from bouncing sand and drifting dust..this can be very rough on moving metal parts. I recently sandblasted my entire 54 Plymouth, out of doors..two stages..first half was with zero wind and the second half it was tad gusty at times..I lost about 10 gallon of media for the total job but such a small price to pay for such a nice de-paint. The sand was recyled from my cabinet blaster so in effect I more than made out cost wise. I will admit to reclaiming the material a few times during the process..I used a large tarp and strained the reclaimed media a few times as my pot was but a 40 pounder...

[PatS....]

There are some detailed in depth threads on sandblasting at Hotrodders.com, but this linked article was the most balanced I have come across: (scroll down a bit on the linked page for the damage to sheetmetal paragraph)

http://davepropst.com/Article/Art5/Article5b.htm

MUCH MUCH more at the link:

Media Blasting Damage of Sheetmetal Panels

When media blasting metal castings, heavy fabricated metal structures, or any such more or less indestructible items, if the process is not proceeding at a reasonable rate changes can be made to speed the work up. Coarser grit media, heavier media, sharper media, higher air pressure at the blast nozzle, greater volume of flow at the blast nozzle, or any other more aggressive means of blasting all serve to speed up the stripping process. This is generally true no matter whether paint, rust or some other substance is being removed. In the case of those 'indestructible' parts, this more aggressive blasting is often acceptable because there is little chance of any significant damage to the item. Body panels made of sheetmetal are an entirely different matter though.

Aggressive or even normal blasting (normal for heavy metal parts that is) can cause damage to sheetmetal in a number of ways. Warpage, a too-coarse surface texture, and work-hardening of the metal are very well-known problems. Warpage and coarse texture are visually apparent... as are the problems they create. Obviously, warpage will lead to a very wavy paint job unless it is corrected. A coarse texture will make paint prep work more time consuming. The best solution is to sand the coarse texture off with a random orbital sander. Work-hardening though, is not a visually observable problem. When it occurs, what problems it causes, and how to avoid it are much less obvious than texture and warpage situations.

To quantify how much work-hardening is 'a lot'... most everyone who has done any great amount of body panel dent repair has run across at least a few instances of severely work-hardened metal. This usually is in instances of repairing damage that has occurred in the same location as a previously done extensive repair. If the previous repair person used a lot of on-dolly hammer work, extensive water-quenched heat shrinking, and whatever other deadly deed to achieve the repair, that localized area is often work-hardened to the point of being extremely difficult to repair again (i.e. has much higher yield strength). Enough blasting in a localized area using a very aggressive blasting media/method can achieve similar levels of work-hardening.

Work-hardening, that is to say work-hardening that is severe enough to cause problems, occurs to sheetmetal when the blasting method is too aggressive. As the blasting method is made less aggressive the degree of work-hardening is reduced until it becomes of no real consequence. Aggressive blasting acts much like a shot-peening operation. It serves to increase the yield strength of the metal. Technically, it is affecting only that metal at the surface or near the surface of the item being blasted. However, in light of the fact that body metal is quite thin and in light of the fact that both sides of the panel are often blasted-- the net effect is that a significant percentage of the thickness of the metal is being hardened. The effect can be substantial with sheetmetal whereas it is often minimal with heavy chassis parts of large cross-section. To explain... consider two test strips. One is a length of flat bar. The other is a length of sheetmetal. The overall percentage increase in yield strength of the test strip that could occur as a result of media blasting a 1/2" thick x 6" wide length of flat bar would be extremely small. On the other hand, the overall percentage increase in yield strength of the test strip that could occur as a result of media blasting a 6 inch wide length of 20 gauge sheetmetal can easily be great enough to make dent repair a real chore. The flat bar example contains a high percentage of metal deep within the cross-section that is unaffected by the blasting. Given an equal amount of blasting, the thick flat bar simply does not gain as much (percentage) strength as the sheetmetal. That is because a large percentage of the metal is unaffected. On the other hand, since the sheetmetal strip is so thin, a much greater percentage of the metal it contains is affected by the blasting. Therefore, the sheetmetal part-- the test strip-- realizes a greater percentage gain in yield strength than the flat bar. What this means is that while aggressive blasting generally has little if any net effect on the yield strength of a heavy structural part, it has a dramatic effect on the yield strength of light-gauge sheetmetal.

[Don Jordan]

I went to the guy doing my car and at first he seemed offended that I would suggest such a thing to him. He already knew of the ill effects of blasting and said he's been doing this for such a long time that he was aware and took precautions. It's going in next week. In the mean time I've got to start rounding up parts. The problem is the guy working on my car doesn't seem to realize that I actually have to work and can't spend all my time chasing parts.

this should be another thread but let me sneak it in:

Chrome!! I probably should re chrome the bumpers. It seems to be harder and harder to find someone because of the environmental issues. I found one place that will do a bumper for $300. How does that sound?

[martybose]

(snip)

Chrome!! I probably should re chrome the bumpers. It seems to be harder and harder to find someone because of the environmental issues. I found one place that will do a bumper for $300. How does that sound?

If the price includes preparation and minor repair, and is a quality chrome job, it's not too bad.

I've got a pair of bumpers on my car that I would like to chrome; ask him if you can get a quantity discount!

Marty

[Normspeed]

Sounds pretty close Don. I've been quoted $250 per blade, guards extra, by a couple SoCal chromers. Some shops take their work south of the border where enviro laws are looser or are not enforced.

[PatS....]

I went to the guy doing my car and at first he seemed offended that I would suggest such a thing to him. He already knew of the ill effects of blasting and said he's been doing this for such a long time that he was aware and took precautions. quote]

Better to check now than wish you had afterwards I think.