Sandblasting the body questions.

sidevalve

sidevalve

Senior Member
I have a 1929 sport coupe that I'm working on. Tonight I have a sandblaster coming over to look at the body to clean it up for patch panels and painting.

The bottom of the body is where it is going to be patched in the usual places.
The rest of the body just needs to have old paint stripped off.

I have heard that sandblasting the sheet metal can warp it. So my question is: Should I just have him spot blast the rust off and not blast the sheet metal?

I haven't met with him yet to see what he thinks or to ask exactly what type of sand or media he uses.

Any input of what questions I should ask him would be greatly appreciated. Thanks, Matt.
 
Be very careful with sand blasting. It wil warp sheet metal very easily. Your better bet is to ask him to soda blast the body. You will actually get a better job with minimal to no distortion.
 
Technology in blasting has come a long way in the past few years. Interview him and ask what kind of media, and at what pressure does he plan to do the work with.

I like to use plastic media at 40 lbs (+/-) to strip paint, and aluminum oxide at 35 lbs to remove rust. At those pressures, there is very little possibility of warpage. IMHO, it really does not matter what the aggregate is (soda, CO2, aluminum, glass, etc.) as long as the pressure is low. Gone are the days necessary for them to use sand at 90+ pounds of pressure to strip your body.

.
 
I agree with Brent but would like to add a few cautions. Even with the pressure low at 35 pounds one must be very careful with the loovers on the hood. I would also be careful with the center areas of large pieces such as the hood, doors and the center of rumble or trunk lid. It has been my experience that it is better to approach those areas with the medium hitting the metal at a very shallow angle. By doing the removal this way, one can actually take the paint off almost a layer at a time. Once close to bare metal, that may be the time to switch to a DA. Gar Williams
 
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Yes sandblasting can warp the metal--- if done wrong.

It does so by stretching the surface of the metal like little hammers. So the one side of the panel gets a bit longer.

This is not done through heat as many guys will try to tell you.

There are two ways the panel can be warped.

First is because of too much pressure. The little bits of media are thrown at too high a speed and cause little dents that can move the metal a tiny bit.

Second can be from too much media in the stream. One small hammer does not hurt when dropped on your head, but a 1000 of them will hurt.

Having done 4 cars with sand and never having a warped panel I have a bit of experience. Using a Model A smiths compressor head running at idle giving 40 PSI at the pressure pot and the stream opened up until you just see the media in the air stream.

Knowing what I know now I would take different approaches for blasting. Certainly any areas that are curved or inside or any areas with rust I would blast or try electrolytic rust removal. Flat areas I would consider using Norton 80 hook and loop sanding pads for paint removal.

Now it is important that all the pits have the little dark spots removed when blasting. That dark spot will cause rust to bloom out rapidly in the right conditions. Oh and NO you can NOT fix that little rust pit with various acid etches, contrary to what the labels say. I have experimented and found they can not stop the dark pit from causing troubles.
 
I have been in the blasting game since 78.

Do not worry about pressure.

Worry about type of media and size of compressor/nozzle.

I could blast hood louvres with 200 PSI and fine media and a 1/8th inch nozzle, and a 60 CFM compressor, but would destroy it with coal slag grit and 90 PSI with a 350 CFM.

Never blast a shallow concave panel or a dead flat panel or any inside panel with a reinforcement rib .

Soda does not remove rust and is too smooth a finish anyway .

For most of my career, I used a 175 CFM at 100 PSI and a 5/16th venturi nozzle and ilmanite, which is a fine black mineral.

For instance, it is quite safe to blast a model A door in the jambs and outside, but even a dust over the inside skin and the blast goes in the holes and hits the inside of outer skin, it stretches the skin inwards and warps it permanently.

I have successfully blasted aluminium bonnets with 175 CFM and 100 PSI.
 
It may be worth the ride to Milford, Ct. to have American Dry Stripping do the work. They have been in the business for 30+ years and all the pro shops use them. They offer an enclosed trailer pickup and delivery of your car or parts. I'm not connected with the shop, just a very happy customer.
 
Back in the 70s, when I restored my 29 coupe, I removed the paint with chemical stripper and a putty knife. Then I has the sandblaster just go over it lightly and mostly on the door jambs and other detail areas.
 
sidevalve said:
I have a 1929 sport coupe that I'm working on. Tonight I have a sandblaster coming over to look at the body to clean it up for patch panels and painting.

The bottom of the body is where it is going to be patched in the usual places.
The rest of the body just needs to have old paint stripped off.

I have heard that sandblasting the sheet metal can warp it. So my question is: Should I just have him spot blast the rust off and not blast the sheet metal?

I haven't met with him yet to see what he thinks or to ask exactly what type of sand or media he uses.

Any input of what questions I should ask him would be greatly appreciated. Thanks, Matt.
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I use to own a sandblasting and painting business in Florida I have done everything from tub grinders to airboat hulls. I have used all kinds of media pressure? well that depends on you type of nozzle and cfm,s if the guy doesn't know this I would not let him touch my car. Plastic media, sodas , walnut hulls can be very expensive to have done. you have to have certain type of dryers and raises the cost. I can tell you a trick that is very inexpensive media that works VERY well I have used it several times also on my 1930 closed cab pickup which I done with out any war-page.
If you have a Funeral homes or someone who cut headstones just get there wore out media which they will usually give to you. just filter it. It is broken down and wont cut the stone anymore. its like blasting with soda it wont dent or hardly even rough up the metal but very good on removing rust. You can even use it with a venture nozzle. Warped panels are caused be tiny dents or in the sandblasting world we call it a tooth the depth and width of the tooth is some times required on government work. And will be checked. Trust me when I tell you your average sandblaster usually doesn't know this and can destroy your car or at least cause you alot more work. hope this helps.
 
This one drives me crazy!! It' s easy strip the body of paint first and then sandblast. Paint stripper is made to remove paint. Sandblasting is made to remove rust. I've cleaned up over 60 cars this way and only once did I get some warpage. That was on a Chevy Fullsize van that was all surface rust. The only spot that warped ws ahead of the rear corner on the top of the 1/4 panel. Flat panel, What do you mean flat panel. J.Poole
 
Flat panel is just that...flat.

Sandblasting stretches metal, so a flat panel is stretched by the blasting and blown inwards by the force of the air.

So it does not recover and remains warped.

One should only blast a convex surface where the stretching is raising back up towards the air stream, and the natural original stretch on the outer side of the metal from the pressing dies overcomes the blast stretch as you pass over it and it relaxes back into shape.

It is OK to blast severe concaves like the inside of a fender crown, but I would be very wary about the inside of a roadster quarter and the inside across back of a tourer for instance unless the blaster had plenty of experience.

Once you feel any sort of oil canning, you are probably ruining the panel.
 
59 ole fart.

And have almost got my first ever rod on the road.

Had restored cars all my life but am a bit tired of being almost run over and off the road, so now will be able to keep up with them.
 
I was a machinist working for a guy who loved cars. Victor found and retrieved a 1920 Essex Touring car out of a barn in Vermont. He got the thing home and I made it run and took the running boards and fenders off. Victor Wanted to have a friend sand blast them. They came back all twisted and distorted. I don't know what equiptment or media was used, but it did a job on those fenders and R.B.s. This was no Mod A, there were about a dozen 1920 Essex Touring cars known at the time. I moved on to another job soon after and never did learn what happened to the car.
Terry
 
Tinbasher is right ,strip the paint off first and sandblast the rust. I blast with 00 sand when i can find it or fine black beauty. 40 lb. pressure all day long. I never shoot straight at the spot always on a angle. And always metal prep after and followed by paint thinner .
 
BRENT in 10-uh-C said:
Technology in blasting has come a long way in the past few years. Interview him and ask what kind of media, and at what pressure does he plan to do the work with.

I like to use plastic media at 40 lbs (+/-) to strip paint, and aluminum oxide at 35 lbs to remove rust. At those pressures, there is very little possibility of warpage. IMHO, it really does not matter what the aggregate is (soda, CO2, aluminum, glass, etc.) as long as the pressure is low. Gone are the days necessary for them to use sand at 90+ pounds of pressure to strip your body.

.
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Brent,

So what your saying is I can use AO in lieu of Starblast and reduce my operating pressure from 60 70 #'s to 35#'s. I am using a 40# pressure tank style blaster. This may give me longer cycles of blasting. What aluminum oxide material are you using??
 
Use CT dry Stripping in CT. They do it right! You won,t be sorry.
 

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they are very good BUT $$$$$ .i bought a new compressor , sandblaster 80 lb. sand holder , 2 sets of top of the line hoses and allot of gloves , tips and 800 lb. of black beauty ....it was cheaper doing that then having them do a frame, 5 wheels and front end and rear end.
 
The size of the nozzle has a lot to do with it. A commercial setup with a large nozzle can cause a lot of damage if not done correctly. But a home/hobby size blaster with a small nozzle is fairly safe. I have blasted sheetmetal with no damage. And we are talking very thick metal in this forum, so again the chances are very small.Usually I blast much thinner gauge metal.
 
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