Warping sheet metal - sandblasting

Pooch,
You must be superhuman to do 3 front fenders in an hour. I guess I don't have any skills doing sandblasting. It takes me much longer with my small tip, but I have never warped a piece or sheetmetal that I know of. I wish I could do a good job in 10 minutes a side, but it takes me much longer. I am retired, so my time isn't all that valuable.
Rusty Nelson
 
700rpm said:
You read the warnings and you proceeded anyway? Why? :confused:
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I guess you missed my comment where I initial thought that heat was the reason it warps. So I was keeping the panel cooler than when wire wheeling.

Like many have said, there's nothing better than blasting - I agree. I just need to learn the skills these other guys described - practice practice.

This is the best way I learn - study, try, practice, etc. I think Edison did something like this.
 
Most of that hamb thread was in trying to repair the damage done.

The measuring and info on the blast damage and heat was good.

Every panel starts off as flat sheet.

When pressed, the outer side is stretched and inner is shrunk.

The middle of the steel is the same length as it was before pressing.

When the outside convex is blasted it is stretched but has all the surrounding metal already in stretch to pull it back.

When the inside of a concave is blasted the shrunk side is stretched again, and the surrounding metal has not got the properties to pull itself back into compression.

The outer blast is over stretching the already stretched side, so it has more pulling power to pull it back.

The inner blast is stretching a compressed side, so bringing it back to what to what it was naturally before pressing, so the inside is relaxed back to normal and the outer stretch has not the pulling power from the outside to re-compress the inner side.

This is highly accentuated when the blast is passed over a rib or in the case of that trunk, a hole.

All around the very edge of the hole, the metal is stretched badly, as the surrounding metal is protected under the inner frame and has no pull back properties at all, so the stretch is worse at the very edge of the hole.

The oil canning is because you end up with both sides stretched, or more than likely the outer side still stretched from pressing and the inner side stretched back to original or stretched as well, so this makes the panel skin longer than it was on both sides , and metal has to go somewhere, so it waves.

This is why I warn about areas on a model A... like blasting the inside of a door and if the blast hits the outer skin through the holes, it can stretch and suck the outer panel in, and you will see four hole imprints on the outer skin, even though the holes are an inch or so from the outer skin.

Same with the angled front floorboard riser against the side if the cowl.

Blast there with any aggression inside and you will see a suck in blast line on the outside of the cowl.

When I damaged my first hood, after blasting both sides, it took me a while to figure it out.

It for sure looked like it was blown in from the outside, with all the inner frame holes/shapes showing on outside.

Next hood was taken a lot more carefully, but it was almost instantaneous, and I discovered it was stretching that caused it.

This was in 79 and before the advent of the net.

Most of you guys will not get into trouble using piddly air pressure and a 1/8 th nozzle, but as long as your industrial blasters know about stretching, you should be OK.

If you want to experiment, get a piece of scrap with a very slight crown on it and lay a scrap of steel on the inside and blast along its edge, and then give the outside a file, you will see the rule imprint sucked in where it was blasted.
 
Kevin in NJ said:
No it is not due to heat. The sparks you see are from individual pieces of sand breaking. You do know that quartz is squeezed with a snap to generate the electricity to light your grill?
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Oh, that's cool. I didn't know that the light generated was from the silica sand breaking up. Thanks for the info.
 
mramc1 said:
Oh, that's cool. I didn't know that the light generated was from the silica sand breaking up. Thanks for the info.
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In re-thinking what I wrote there should be more to it.

There are several ways to create sparks. It is all about energy. You are putting energy into ripping off pieces of rust too. I would think the break up of materials will generate point heat that make sparks. Much like you get sparks from a grinder ripping off bits of metal.

I know some of the sparks are from the sand breaking. Some are more yellow and I am guessing that we see some just from the surface rust breaking off.

So I can see there would be more than one cause for sparks. In each case it has to do with small pieces breaking and the energy released in breaking them.
 
The Trick to sandblasting is to strip the paint off of the panel with paint stripper and then use the sandblaster to remove the rust. The biggest mistake people make is they think they can sandblast paint off the surface. It take a lot of force to remove the grip of the paint on the surface and you have to stay in one place to long which creates heat which causes warpage.
I say it again!!! The Old Tinbasher
 
Tinbasher said:
The Trick to sandblasting is to strip the paint off of the panel with paint stripper and then use the sandblaster to remove the rust. The biggest mistake people make is they think they can sandblast paint off the surface. It take a lot of force to remove the grip of the paint on the surface and you have to stay in one place to long which creates heat which causes warpage.
I say it again!!! The Old Tinbasher
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Would you do a strip to get it off to clean metal and then blast it?
 
The vehicle I'm working on is off topic but the work isn't,we picked up a 1936 Glacier park bus that has been around the block several times and been left sitting out in the Monterey area, quite a bit of bondo and a lot of holes in a couple panels from someone trying to straighten out warpage from sand blasting,the sides of these buses are alum from the drivers door back to the drive axle and the same on the passenger side,the top,back and front are steel,damage is pretty severe on the side panels from blasting,when the guy that does my blasting took a look at it he showed me what the person before had done,he read the warpage, a couple places it was really pronounced,it looked like one of them gals on home and garden tv painting a house wall,a stroke this way and a stroke that way,the warpage was all over the place,there is no saving of a couple side panels or door panels. Now with all this being said I wanted to soda blast the body and sand blast the frame,this was a body off operation,even as big as these are,there isn't much difference in price between sand a soda but the effects are amazing,yes soda does leave the finish pretty smooth but a self etching primer will take care of that,the other drawback is you have to wash the soda off and out very well or you will have problems down the road,I used a hot water pressure washer to wash the body inside and out,it is just a shell so this was pretty simple, also the hot water would nutrulize the soda according to my blaster. Soda blasting does not remove rust, dirt, grease and grime,so that needs to be cleaned off prior to blasting. As far as far as stripping paint off prior to soda or sand blasting would be extra work, a good blaster can do wonders for getting paint and rust off.
 
The warping you see on sandblasting is caused from the peening action of the media striking the surface, nothing else, and certainly not from heat. Any heat is a SIDE EFFECT of the peening action, and any stretch has already taken place before any appreciable heat is realized. The two primary causes of stretching via media blasting are pressure too high, and media too coarse. Think of it as using a larger hammer and swinging it harder. You can media blast using a pressure washer or do it underwater for that matter, all in an attempt to keep things "cool". It will still warp given enough pressure and media too coarse for use with sheet metal, and you have taken the heat out of the equation. Fix the source of the problem, not the reaction.

Change your media to a finer grit and decrease pressure to about 35 psi or so. Practice on some scrap to see if this doesn't help. One of those large industrial blasters set to about 80-100 psi will do wonders removing rust and scale, but will also beat the hell out of the metal. Lower the pressure...
 
This warping is easily demonstraded by blasting the side of a 1" thin putty knife. I used about 80* air pressure and held the gun about 2" away. The blade curled right around with only a few seconds of blasting. I made it straight again by blasting the other side.
 
My understanding is that soda and plastic media blasting will not remove rust and will only remove things such as paint etc.
 
wrndln said:
Pooch,
You must be superhuman to do 3 front fenders in an hour. I guess I don't have any skills doing sandblasting. It takes me much longer with my small tip, but I have never warped a piece or sheetmetal that I know of. I wish I could do a good job in 10 minutes a side, but it takes me much longer. I am retired, so my time isn't all that valuable.
Rusty Nelson
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No Rusty, when I was full on blasting in the late 70/s and 80/s, I was about the first in Sydney OZ to specilaze in vintage panels.

I used to go to swaps with the typical half blasted hood etc to show before and after.

Now there are dudes everywhere doing it.

I used a 175 CFM compressor at 95 PSI and a 5/16 th inch nozzle.

As you could imagine, in the hands of a beginner, it was way too big, but I had it tamed .

I could do aluminium bonnets (hoods) successfully .

It did take a couple of years to finally get really good at it , meaning bang for buck for customer.

I can just imagine using the 40 PSI and miniscule nozzles mentioned here, it would take a day to do a door !!!!
 
We use a Smiths compressor head on a B block. Basically it sits at idle all day long. We run the air through like 50' of tubing coiled in water then on to bottom of an old water tank to allow the air to expand some. We keep a valve on the bottom cracked open to constantly drain out the water.

The old TIP 50 or 100 lb pressure blaster is kept at 40 psi on the input side. We run a larger hole, I forget the size, as the Smiths supplies a lot of CFM. The sand valve on the bottom is cranked open till you just start to see sand in the stream.

I will say there is a lot to having lots of CFM at your disposale when you need it. I have a 1946 vintage 5 HP compressor at my house. Best $300 off of craigslist.

The siphon blasters, even at lower pressure, can warp the metal because you can not regulate the amount of sand in the stream. Too much sand is the same as too much pressure.

With some years of research and experience I will take a different approach to clearing paint and rust off of a body.

The guys who make money on this stuff will use Norton 80 grit hook and loop with a sander for flat panels. The hook and loop cuts different then sticky back so I understand the difference. Blasting rust and making sure I clear the brown spots in the bottoms of the pits is one of the best ways to go. Now I would also consider using electrolytic rust removal to the point of making panel sized pools. This can take off some paints too.

There are many ways to get the paint and rust off. You need to select the ones that fit your resources. It is important to know the different methods as they each have their good points and bad points. Sandblasting can be one of the more expensive, dirty and time consuming methods.
 
A simple question? Did you strip the paint first? This make a huge difference to keep the metal from warping. I find 90 PSI of pressure works well stay back about 2 feet and use many light passes until yo get it clean. It take more time to repair the metal then to sandblast it so take your time. Even if you do one part a day until your done. Go easy on the flat panels. The more shape in the panel or the more lines, it can be sandblasted faster because its stronger.

The Old Tinbasher
 
I agree with Tinbasher . Stay back and make several passes moving all the time . If you move too slowly or stay in one spot very long you will warp the metal or put pin holes in the rusty spots.
 
Joe K said:
Plus one to all of the above.

One of the marvels of my engineering experience is "pein straightening" which is the use of a rather dull impact tool placed against a shaft and "hammered" with a device like a chipping hammer - but making no discernable marks or hammerblows on the surface of the shaft.

Contrary to expectation, the tool is placed not on the "high" side as in effort to push the shaft into straightness, but rather on the "inside curve" of the bent shaft.

I saw a boiler feed pump shaft that was fully 1/4 inch out of straightness brought back into 0.000 by the use of such a tool.

Another advantage of pein straightening are it stress relieves the shaft as you go along - existing stress, such as caused by the impact or whatever that bent the shaft - are "erased" by the sonic shock wave that such straightening creates.

It also does not change the diametral dimension of the shaft.

As you do it, you can literally see the shaft "walking" towards you.

When the shaft is straight - one can be assured that no residual stress will remain that will cause the shaft to act "funky" under increase in temperature or normal stress of operation.

It's magic.

I marvel.

Joe K
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Great lesson I have done so in the past.
 
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