Sheet metal sandblast damage - a test subject

One of the guys on another forum I frequent had the misfortune of trusting his 55 Chevy to a sandblasting expert ("been doing this stuff for thirty years!") and now is trying to regroup from the damage. The trunk lid was only sandblasted from the inside, and needless to say, too much pressure, too coarse media, one or both produced these results......



.......where the peening action caused the inside of the skin to stretch until it bulged inward. Discussion ensued as to the correct action to take to remedy the situation, with the primary differences of opinion being:

1) sandblast the outside to equalize the pressure caused by the peening (that caused a stretch on one side only) and then once equalized, shrink as needed to restore the panel to proper crown.

2) Shrink the panel as it sits, then take corrective action (stretch/shrink as needed) to restore the panel to proper crown.

After various inputs, the owner of the car is rather timid to try anything until a concensus is reached. So in the interest of learning something, I decided to help the guy out by conducting a test of both theories, and see what plays out. A local test subject was obtained, and also picked up a bag of medium grit Black Beauty from United Rentals.





I got a chance today to sand all the bondo off the top part of the sample trunk lid. I took some measurements, made a template of the inner brace opening we will be using, and laid out the pattern on the outside so we can monitor the progress of the different methods. A datum was drawn about 2" in from the forward edge and the center edge of each hole. This was chosen as it is approximately the center of the radius on either end.





Next, the panel was checked along the datum lines in both directions to give us a baseline for panel straightness, and we'll check along these lines after sandblasting to see what damage occurs. Where there was a bit of waviness, it was a minimal amount that would have disappeared with some high build primer and block sanding. Given our 57 year old test subject, I guess we're lucky it's that close.









And here's our test subject, awaiting sandblasting.



until the next installment...........

 

your attention to detail is amazing!

 

Good luck ,The sheetmetal is stretched inward ,I made that mistake once on a decklid ,I didnt ruin the panel though ,A few skim coats would fix it.But from then on Ill never blast anything from the inside ,Its the heat that warps things...

 

we always have trunk lids dipped, or any other sheet metal that has holes on inside on panel

 

John, thanks for that reminder. I've heard the same thing, then I've heard it is a myth, and I've also heard heat from welding shrinks so how can heat stretch.....

To get an answer I think I'll add that to the test, borrow a digital thermometer to see actually how hot the panel gets.......

 

Dont look that bad, I made a terrible mistake on my 55s trunk lid!!! I would be happy to trade ya

 

A H.A.M.B. Myth Busters episode .....I LOVE it !!!!!!
I'll be following this one closely !!

 

Interesting, Very Interesting, Must have used way to mush pressure (65 lbs and #1 washed sand 1/4 inch nozel )blast at 18inchs away or more ,dont stay in one spot to long. And never never blast a flat panel .

 

Why blasting sheet metal with medium grit anyway? I always use fine/extra fine and have no such problems...

 

I need to duplicate the damage first, so I can try fixing it using different methods to see what actually works best. That's why the medium.

 

You could speed the repair process by posting a thread asking for help from any of the many sheet metal experts here.
But if you wanna kill loads of time and 2 trunk lids, who are we to stop ya!

 

yea my dad had a guys car in the shop that "had" to have the insides of the front fenders blasted, the guy ruined a damn near nos set of 57 chevy fenders. my old man made him get him another set , they were so fucked up...

 

Watching for results very closely here. Had quite a few cars/parts messed up by improper blasting procedures.

 

Hey Mac,

That's some pretty ''heavy science'' you have goin' there! Your '' highly calibrated linear measuring device '' reminds me of the old Earl Schibe ''patented Dent-O-Meter'' used to estmate collision damage in the Schibe paint shops.

I've gotta tough time with the concept that replasting the panel from the exterior will return the panel to an unstretched state. How would ya figger the same pressure, distance, grit used on the original pass and whether the panel had any old damage i.e. stretched or areas under tension?

The second option of the use of a shrinking disc & some lite bumping/dinging with a spoon/slapper would be the way out. Even the use of a small torch shrink and a sharp vixen file drawn across the shrink to tighten up the panel. It's the pressure of the plast against a ''low strike'' low tension, low crowned panel that caused the stretch, not the heat.

Thirty plus years ago I restored a '58 Cadillac & had all the dog house tin, door shells & deck lid dipped. I figgered, what a great way to kill off the rust I couldn't see & remove all the old finish all in one shot...........................jeese whata dumb idea! Five years after that job left the shop rust and paint problems continued to haunt me everywhere there was a pinch weld or seam. Eventhough the panel were ''neturalised'' washed & sealed with primer.

Today I'd still have panels blasted, but not by some guy who's use to blasting pipe fittings & valves! Low crown panels require sharp grit @ low pressure & never at a ninety degree shot to the panel.

Good luck,Mac

" Meanwhyle, back aboard The Tainted Pork "

 

subscribe.
Dave

 

Great setup! I can't wait for the results.

 

I don't think he meant sandblasting the outside would render the panel "unstretched", but rather wanted to relieve any stresses prior to any attempts at shrinking. Personally, I would think more stretching, regardless of choosing the opposite side, would push a bulged panel even more into it's bulge. But we'll try it and see what actually occurs.....

 

Interesting...Very Interesting .can't wait to see results...

 

Well here's phase two of the test. The trunk lid was fitted into the blast cabinet.



You always hear about how the heat from sandblasting is what causes the stretching and warping of the metal. I never gave that theory much thought before, but while we are testing, this would be a good opportunity to add that in the mix. In an attempt to minimize any outside influence while we measured temperature, the flood bulbs inside the cabinet were unscrewed enough to keep them turned off...



Here is the thermometer we'll be using today....



And here is the baseline temp.....



This is the media we'll be using today, Black Beauty medium grit. A bit coarse for sheet metal, but we're trying to duplicate the damage seen on Joe's trunk lid, so we'll give it a go.....



Then I changed out the media in the cabinet, and started blasting. As I had opened the garage door and it's rather warm out, and I was rather slow in getting the media changed out, I thought I should check the temp again.



After clearing off the one hole, we had noticeable panel distortion from the outside. In checking the temp again, it had not moved since the last check.



So in essence, the temp of the panel did not change from the time we started blasting the one hole until it was finished. If you want to scream conspiracy, well it has gone up a whopping 1.8 degrees since the trunk lid was placed in the cabinet. I doubt seriously this can be attributed to a heat issue. I do have a refrigerant dryer on my air system, so that helps to keep the compressed air from getting warm, and yet the panel stretched still, despite the abscence of any significant heat increase. It should be safe to say the damage is caused due to the peening action of the media against the panel surface. Stretching has long since occurred by the time any heat has finally shown up.

Lastly, here is the noticeable damage to the panel, measured along the datum lines as before....









Just to show a comparison before and after........









Next phase of the test will be to sandblast one of the damaged areas from the outside to attempt to relieve the compression/stresses of that side of the panel and see how the panel reacts... I'll use our driver's side and we'll monitor what happens.....

 

fascinating...

 

Just to say something on your test ,Originally you said sandblasting and did you change to the exact media,,,,And for you guys out there that do it yourself I was blasting a 65 malibu decklid for a job Made quick passes on the underside of declid where there was no reinforcement And they pulled ,I used triple 000 sand The owner was oissed ,And I quit a few days later ,The bodyshop wasnt worried though ,A few skim coats no big deal.Owner didnt want filler on it,,,,

 

John, I don't know the exact media he used, but this is the machine in action:





It's hard for me to compete with that using a siphon feed Skat Blast cabinet, so I may have used slightly more aggressive media to try and achieve the same damage results.....

 

This.....is......awsome.


Thanks for taking the time to post all the pics, write the info, etc. I can't wait to see the solution / solutions.

 

the heat factor always spins me out when applied to this situation. you are shooting a bus load of mostly cold air and chosen abrasive at the item, you would have to hang around in one spot for quite some time to build any real heat to cause damage. stress relief is what is going on,no doubt. i ruined a hood ( or as us aussie guys would say bonnet) by blasting the underside of it for a customer. i told him probably not a great idea but he insisted. i shot it from at least 3 feet away and just gently waved over the whole thing, not trying to blast it clean in one pass either, just gently taking the paint etc off in multiple light passes. keep in mind i was only using beach sand through a 5/32 nozzle on my small pressure pot at around 100psi. also , out of a nozzle that small and at the distance i was from the panel, the spray pattern so to speak was fairly broad. as i told him would be the case it was warped, you could clearly see the outline of the inner frame in the outer skin. it had pulled back toward me while it was being blasted. if their was no inner structure maybe the damage would be spread over a larger area and not so noticeable. i generally just stay away from low crown panels with the blaster and use other methods....like sit back and have a beer or dozen or so and think of other things for a while

 

Cool thread, you have my attention.

 

Im following this one also .It might not be heat causing the warpage ,If you take sheetmetal and Hit it with a hammer on a flat surface ,The edges will curl up.That might whats been happening and we blamed it on heat for years.....Love this thread As a matter of fact Im blasting my front end tomorrow ,I dont have to worry about warpage though......

 

On to some "repairs" ?
To preface, both test holes were bulged toward the inside. When I tried to push either out, they would oilcan out, but the stresses in the metal would bring them right back in once released. We started this off by adding some wide electric tape over our length markers. Just in case we want to check again....



The driver's side test hole was sandblasted, and I stayed within the perimeter as marked so as not to stretch anything that was behind the inner panel and thus would be less accessible for any repairs. Once complete, you could tell the sandblasting had indeed relieved the stresses, as it would now oilcan in and out freely, and would stay where you put it.
Pushed in, it does appear that the panel is about where it was before......



Pushed out, it shows some stretch as the panel assumes a sinusoid wave of sorts......



On to the passenger side test hole, this one is going to be addressed using the official Donut Dolly (produced by Daniel Gunderson). These have a concave feature on one end and convex on the other.





The basic principle is that the body hammer is used on the high or convex side, and the dolly is placed against the low or concave side. The dolly, as opposed to typical off dolly work, provides support completely around the hammer strike, instead of just to one side.



In order to keep track of things, I've found it best to lay out identical grids on either side of the panel. Align the dolly on two crossing lines on the bottom, align the hammer strike to the corresponding one above, and slowly move both across the panel as you use moderate hammer strikes, stopping every so often to verify alignment top and bottom......







After about 5 minutes of shrinking, using a uniform pattern within the test hole, the panel now appeared to be relieved of its stresses as the panel would easily oil can back and forth. It appeared well shrunk, as it was a loose oilcan. At this point, I used my fancy hitch pin dolly and switched between the flat faced body hammer and a slapper to stretch things back and restore the crown...





After about a half hour or so of bumping things around, the panel had been restored fairly well to the original crown. It would oil can inward if pressed, but once pressure was released it popped back up again.







A bit more work would have gotten things closer, but how close do you need for a test, right?
Here's the bottom side to show the hammer/dolly marks during stretching...



In using the donut dolly, given the nice loose oil can that resulted, it appears the process of forcing the expanded/sandblasted area into the hollow of the dolly compresses the panel back similar to where it was, relieving the stresses.

While using the hammer and dolly to planish the passenger side test hole, I did notice an ever so slight bulge around the outer perimeter of the test hole on the outside in much the same way as the force from a dent causes a bit of springback and resulting bulge around that dent, toward the opposite side. So part of the repair process was to use the hitch pin dolly on the underside, just inside the edge of the hole opening, and to use the slapper on the topside in an off-dolly to help bump this back down. As with many dent repairs, there will be shrinking done from both sides of the panel.

We'll have to see if we can use a different shrinking method for finishing the driver's side and see how that reacts.....

 

This is Cool! Cannot wait to see the results of blasting the other side, and then how you actually fix it.

 

Blasting outer sheetmetal is a no-no in my shop, period. Lots of bad experiences with it. This is good subject matter for everybody!

 

I was just reading through the different comments this morning from the original post to see what if anything we may have missed, or anything that may need further clarification. I did find a few things to bring up for discussion...

I read that part and then, in reading the part highlighted in blue, I was wondering why the donut dolly seemed to work, both in shrinking the panel, and in relieving stresses, as seen by the loose panel oil can. I glanced back at my drawing I did of somewhat exaggerated damage resulting from the sandblasting, and thought of this:



As a result of the damage to one side only, the inside has stretched, and the outside, unchanged, would be in compression by comparison. (likely until there is so much stretch involved, if the blasting had continued, that it would return to a relaxed/it's original state.) Moving on to shrinking, as the donut is placed against the concave part of the arc, it supports along the perimeter of the dolly and helps to "trap" the metal where is sits. As the hammer strikes from above, you are forcing the sheet into that hollow. Where the inside of the skin is stretched, and the outside is not, by forcing more length of the inside of the skin into the same distance of the outside (i.e.: into the hollow of the donut) are we not in essence equalizing the differing forces that exist on opposite sides of the panel?

The sandblasting of both sides has shown to relieve some of the stresses of the differing forces, as we did show the oilcan is now more equalized in force, it will stay where you push it to without popping back to the original position.

Where the driver's side test area does still show a stretch in the sinusoid wave, I don't think the stretch in this type of surface feature can be effectively removed using the shrinking disc, as the highs (in reference to the side the disc is working) that the disc is looking for alternate from one side to the other, where the inner structure may get in the way of effective work from that side. Perhaps that side could be pushed and held somehow toward the outside of the trunk lid to promote better access for the disc. As I've had limited exposure to this tool, I'll defer those thoughts to someone else with more experience to answer....

While we did not attempt the hammer and dolly to relieve stresses as part of our experiment, I think the repaired passenger side could serve to investigate this theory, to see how the panel reacts. Still have media left, so I'll proceed with this so we can see how the differing stresses will react, if the hammer and dolly will help to equalize them..

Having shrunk the one side to a loose panel oilcan, it did planish back out fairly easily to restore the crown. I think as long as your heated torch spots stayed well away from the edge of the hole opening, where you don't risk shrinking under the inner structure with no access for planishing, then you should be fine using a torch. As with all processes, consistency should be a focus..

Very true, the progress needs to be closely monitored to better know when to stop or move elsewhere.. And don't let tunnel vision keep your focus on the damaged area, monitor all the surrounding panel for any changes/damage..

I've got a shrinking disc on the way, to use on one of the remaining stretched areas. Once I re-damage the passenger side for the hammer and dolly "equalization", that would even give me one sample to use with the disc, one with a torch.

Comments, interpretations, etc welcome......