Warping sheet metal - sandblasting

[AlanD]

Ok, I have read many times that sandblasting sheet metal causes it to warp, and I proved it because I did it. But I want to know why it warps.
I was always told that it warps because the heat generated in the metal from the sand blasting on it. This is where I loose belief.

I was using a small harbor freight blaster on a 55 degree day, so the metal is staying pretty cool, certainly not hot. Now on the same day I used a wire wheel on another piece of sheet metal and got it almost too hot to touch, but no warpage. I should get as much or more warping from the wire wheel, but it doesn't. Why not?

[Sparky]

I think it's due to a "shot peening" affect. The metal gets compressed on the blast side, so it warps. It's got nothing to do with heat.

[MikeK]

Media blasting particles blown with sufficient (too much) pressure will dent the top surface of the sheet. The moved metal is pushed in several directions, including sideways. The cumulative effect of the sideways compressive forces from all the impacts causes only the blasted side of the sheet to expand, bending it.

[mramc1]

I believe there is heat generated in the blasting process because you are abrading material under high pressure. I have blasted in low light (evening) conditions and the rust particles coming off have a dull glow almost like grinder sparks, but not that bright. There has to be some heat generated to cause the particles to glow. It's so concentrated that it doesn't have enough effect to raise the entire panel temperature significantly.

[Joe K]

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

[Bob Bidonde]

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Sand blasting, shot peening, hammering and the English wheel all cause a compressive residual stress on the surface of the metal being worked. This compressive stress causes the metal to curl up (AKA warp).

The compressive stress also has the effect of work hardening (embrittling) the material, more so in non-ferrous metals such a aluminum, copper, brass, etc., than in ferrous metals like steel.

The two effects, compressive residual stresses and hardening make it impossible to completely eliminate the curling / warping without special heat treatments.

[28ACoupe]

It should be noted here that just because a piece of sheetmetal is sandblasted, doesn't mean that it WILL warp. If the blaster is skilled and knows what they are doing, it is a very effective way to remove rust, paint, etc from metals.

I have had all my projects sand blasted for the last 15 years with no warpage to sound metal. However, I give very specific instructions to the person doing the blasting beforehand.

[AlanD]

I must not be skilled. Whats the secret?

And I appreciate the above posts and explanations. Peening makes sense.

[Tom Wesenberg]

To demonstrate what Mike said in #3, after I first built my big sandblast cabinet, I blasted an old 1" thin blade putty knife. The blade curled over at least an inch in just a few seconds of blasting. I held the nozzle close and had about 80# pressure. I flipped the knife over and straightened the blade the same way.

[28ACoupe]

AlanD - What I have the guys do is hold off the metal starting at about 2' at a 45 degree angle or more to the surface and come in closer slowly till the media just starts to remove the surface material. It takes a bit longer to do it this way, but it saves me countless hours of fixing the warping in the metal.

This method has worked every time.

[Kevin in NJ]

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?

To cause heat distortion you need to get up into the 500 degree area where metal turns blue (might be a bit off on the temp, but close enough). Expanding air takes heat out of the metal. What does air conditioning do? It compresses a liquid and lets it expand to remove heat. So you are cooling the metal in one respect. You are also compressing the air against the metal but let us not confuse things. I suggest you blast the panel and feel it immediately. If you got it hot enough for heat to warp it then you should burn your hand.

Sand blasting is best thought of a tiny hammers impacting the surface. The amount of energy the sand can impart into the metal depends on the velocity of the particle or the number of particles at any given time. Think one light hammer dropped on your head slowly is not so bad. One light hammer moving fast is going to hurt. Now what about a box of light hammers dropped slowly on your head, ouch that will hurt bad.

So you can cause damage from too much velocity (air pressure) or low pressure and too much sand in the stream. So you need a pot that lets you control pressure and the amount of material in the stream. These cheap siphon blasters allow too much media in the stream and will warp a panel.

From experience of blasting several full cars including a thinner French metal and not warping the metal (even when trying) I can say it is easy to get great results. We have always run 40 PSI (from a Smiths A head conversion) through a pressure blaster and just cocked the media lever on the bottom till I can see media in the stream. It is slower going, but I know I have never ruined a panel.

So your solution is to experiment with pressures and quantity of media in the stream till you find what works for the media you are using.

The warp is caused by too much energy displacing the surface metal. In effect making once side longer then the other. Very hard to correct. Normal stretching with say a hammer is through the full thickness of the metal. So you shrink the whole thickness. But you can not shrink just the one side. You end up shrinking the full thickness so now the other side is short.

[wrndln]

I have sandblasted many, many model A parts and have never seen any war-page of the parts I blasted. I normally sandblast sheetmetal at an angle and use 80#'s or less with a small tip -3/32". In my opinion, it is the best way to clean metal of paint and rust and give an excellent surface for an epoxy primer, like DP 90. If one were to sandblast sheetmetal perpendicular to the metal with a big tip at 90# of more, there might be some warping. I plan to keep sandblasting sheetmetal and other parts as it works better for me than electrolysis or other methods of removing rust and paint I have tried. I use fresh air breathing system and coveralls, which makes the job tolerable. Just stating my experience.
Rusty Nelson

[pooch]

The metal is expanded not compressed on the top side being blasted.

The surface , if flat or concave, which is most of what auto panels are, will rise towards the nozzle.

A dead flat panel may move in with the air pressure, but it will want to raise initially, but it could oil can, but there are very few dead flat panels on a car, the hood louvres would be the ones to watch most.

The trick to not warping is to know just far you can raise the metal within its normal spring back structure.

Too much raising and it will stay warped, with the correct amount for efficiency and it not stay warped.

I used to use 95 PSI and a 5/16th nozzle and after 3 decades of blasting, I knew when a panel would warp and one that would not.

The worst I have ever found on the exterior of a car was a VW beetle door.

Because it is turned at edge only 90 degrees, and spot welded, they always warped just in from edge.

A usual door skin is folded right over and spotted, it has residual tension and strength to resist any warpage.

And never blast a concave large panel or over an internal rib.

You may get away with inside a Model A tub across the back, but go careful.

Concaves inside a fender or cowl is OK, as these are tight curves with plenty of resistance to raising in.

Inside door skins are very susceptible to staying warped in too.

[crumbaa]

WOW another timely topic. How about soda blasting rather that sand, oxide, or slag media? I am under the impression that soda is much more gentle and produces a smoother surface. Is soda less apt to cause warping of thinner materials?

[wrndln]

Tony,
I don't think getting a smoother surface is necessarily a good thing. Assuming you are going to prime the part that is sandblasted, a little rougher surface will give more "bite" for the primer. Soda blasting is probably OK, but I suspect it is more expensive and takes longer to perform. I have never sodablasted before, so I realy don't know much about it. I will continue to sandblast all my rusted sheetmetal panels.
Rusty Nelson

[700rpm]

Originally Posted by AlanD

Ok, I have read many times that sandblasting sheet metal causes it to warp, and I proved it because I did it.

You read the warnings and you proceeded anyway? Why?

[Ross/Kzoo]

Originally Posted by crumbaa

WOW another timely topic. How about soda blasting rather that sand, oxide, or slag media? I am under the impression that soda is much more gentle and produces a smoother surface. Is soda less apt to cause warping of thinner materials?

Before you go that route read the negative reviews before you proceed.

[karasmer]

How long does it take you guys to blast something for example a front fender? I have little experience except on a frame and it took forever.

[wrndln]

I have sandblasted five or six 28/29 front fenders with a small tip (3/32"), of course the tip slowly enlarges to about 1/8" then I replace it. Front fenders have a lot of surface area when you sandblast both sides. With a small tip it take a while - maybe 3 hours, give or take an hour depending how bad the rust is. If the rust is really bad it may take a little longer. I have sandblasted a front fender in 2 hours when there is mostly paint and just a little light rust. BTW, you need a large 2 stage compressor to sandblast well. You also need a a good moisture trap (or two of them) or the sand will clog up a lot - that is real aggravating. Hope this helps.
Rusty Nelson

[pooch]

Originally Posted by karasmer

How long does it take you guys to blast something for example a front fender? I have little experience except on a frame and it took forever.

It all depends on pressure/ size of nozzle/ type of media and skill of operator.

I used to do 3 fenders an hour.

About 10 minutes a side.

[wrndln]

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

[AlanD]

Originally Posted by 700rpm

You read the warnings and you proceeded anyway? Why?

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.

[Flop]

heres one hell of a write up on it via the HAMB .

Warping , Temperatures , Tests Etc


http://www.jalopyjournal.com/forum/s...d.php?t=658008

[pooch]

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.

[mramc1]

Originally Posted by Kevin in NJ

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?

Oh, that's cool. I didn't know that the light generated was from the silica sand breaking up. Thanks for the info.

[Kevin in NJ]

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Originally Posted by mramc1

Oh, that's cool. I didn't know that the light generated was from the silica sand breaking up. Thanks for the info.

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.

[Tinbasher]

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

[Ross/Kzoo]

Originally Posted by Tinbasher

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

Would you do a strip to get it off to clean metal and then blast it?

[ABento]

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.

[MP&C]

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...

[Tom Wesenberg]

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.

[Marvin/TN]

My understanding is that soda and plastic media blasting will not remove rust and will only remove things such as paint etc.

[pooch]

Originally Posted by wrndln

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

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 !!!!

[Kevin in NJ]

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.

[Tinbasher]

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

[Purdy Swoft]

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.

[jimvette59]

Originally Posted by Joe K

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

Great lesson I have done so in the past.