Sheet metal sandblast damage - a test subject

In your opinion, can you expect this lid to be metal finished to not needing filler? ...and if you think it is possible, can you spray some paint after the final metalworking part to show how close the lid is, to "wave free", before nitpicky blocking/highbuild primer.

 

Hey Mac,

I'd go with your panel spotter shrinking tip, just as you used on the tail gate rebuild, to tighten up this panel. Since this isn't really a heavy stretch, as you'd find in the middle of a quarter or door skin where a bumper tip or crash pole dragged alot of material out of alignment, with thick and thin stretched areas, I think it would work well. I'd go with a circle pattern starting from the outside in, from inside the deck lid. A wet rag dragged across the shrink, just as you remove the shrinking tip would draw in the metal fast. Shrink, check the movement, hammer/slapper & dolly any out of alignment metal, & shrink again until all metal has been returned to normal contour.

Another great post!

 

That would depend on the place that is repairing the damage. For example, the local body shop doesn't have time to do a repair to a strictly metal-finish state, and would be losing money if this were an insurance job. If the damage is where you have access to both sides for all of the damaged area, then I see no reason someone couldn't take it to that level of finish, given a customer with the means and desire to fund such an endeavor. As this is merely a test, I don't see me taking it to that level of finish, but IMO it is entirely possible.

 

Time for the next phase of the test. With the passenger side repaired with the use of the donut dolly, we sandblasted the inside once more to use for another test.



Sandblasted, the panel once again showed a bulge to the inside.... A test of the oil can effect showed it would pop toward the inside once released.





Now we'll use a hammer and hitch pin dolly to attempt to stretch the outside skin to equalize some of the stresses. The hitch pin was pressed against the inside to raise the panel as the hammer was used from the outside..


Once complete, the oil can effect seemed more equalized in that it would stay to which side you pushed it. The panel also shows the sinusoid wave... indicating a stretch within the opening on both sides...



to attempt the repair to this side (again), we'll use heat in the form of the heating tip on the dent puller. Here you go Pimpin'..........





I used the glove to push the panel outward while shrinking with the heat tip from the outside, and the panel just didn't seem to be responding. Then it hit me, the damage was caused by stretching from the inside, so let's shrink from that side.....



This appeared to do the trick, shrinking from the inside. Likely in addition to the weight of the dent puller pushing the direction we want to go, you could see the panel moving slightly outward around the perimeter of the tip as the panel was heated.





This process, with the majority of the heat now concentrated on the inside, was moving the panel outward as it was shrinking. Kept going until the panel seemed to stop responding to the heat shrinks, and then thought we'd planish to bring the crown up the little bit that was needed. Started again with the hammer and dolly, but it did not appear to work as well as it had after using the donut dolly. Keeping in line with the thoughts of the shrinking was more concentrated on the inside, what could we use to keep any stretch concentrated on the outside? Thought I'd give this small shot bag a try as the dolly....





The panel once again seemed to respond well to the tool selection, again bumping the panel the direction we needed.



Not perfect, but looks like we're doing the right thing. A bit more work and it would be to a high build primer fix state.

The care package also showed up this week, so we will try the shrinking disc for its results tomorrow on the drivers side test area...

 

I will stick with my siphon blaster (not that monster being used on the rotisserie car) and 4" plastic paint strippers(black 4" x 1" doughnut looking items) from Wal-mart, it may take longer, cost more, but real hard to booger up metal. Keep all paint stripper away from any seams, BTDT, it ain't pretty 2 yrs. down the road.

 

MP&C you are a ruler thank you

 

very nice read and good documentation and facts. I would like to read more keep it coming.

 

Wow, MP&C...excellent testing and documentation. As B.S. said, you could've saved time by simply asking the tin experts what methods and procedures to use, but you're doing a selfless thing here. You're not only taking extreme precautions for the owner, but also proving out the theories for the rest of us. Thank you for that.

Aside from all the technical info, the one thing this thread has given me...and I'm sure others too...is the realization that sandblasting can be waaaaaay more damaging than I ever thought, especially on low-crown panels. I too, was under the impression that the heat was the main culprit, and as long as the pressure was kept down, and the blasting wasn't concentrated in one spot, it'd be ok. But obviously NOT.

I for one, am going to avoid sandblasting sheetmetal like the plague, for future projects. And I listened intently to the guys talking about the damage that acid-dipping can cause.

I'm gonna gladly embrace the idea of tedious stripping of sheetmetal by hand...no matter how long it takes. Thank you again!

 

But then....half of the "experts" would have said the heat caused the warping. Just like primer first or filler first, one of those never ending arguments.

 

this thread is cool!!!

 

dittos....this is very educational stuff....thanks for taking the time to post all of this.

 

this is a real good thread , actually learn something interesting , teach on

 

Hey Mac,

At this point I'm thinkin'..................ya gotta add '' & Metal Research Laboratory '' to your shop logo and tee shirt!

Again, another great post!

" Life ain't no Disney movie "

 

Why do I need my magazine subscriptions when we have guys like this teaching on the HAMB?....VERRY COOL.

 

I find this stuff fascinating. The explanations are great, I could not grasp how the sheet metal seemed to move in the opposite direction I thought it would.

Very cool tech!

 

I'll be the first to ask a stupid question. Great info BTW, although it is way advanced as far as I am concerned. My question... Is this type of damage, or any damage really, limited to flat expanses of unsupported metal? In other words, I have the doors and interior cab of my truck ready to blast. I was only planning on the jambs and inner panels, inside in between the cowl panels, under dash, where they have beads and bends, welds, etc. My thinking was that the beads and what not would provide the support needed to avoid damage/warping/stretching from blasting. I have blasted a lot of small items using double screened play sand and was ready to pull the trigger tomorrow until I stumbled across this thread...

 

I think the big issue in the peening effect is the size of the projectile combined with the force behind it. Normally a flat or low crown panel is the one that will show the damage the worst, but if you take the same precautions you'd use on low crown panels, you should be safe anywhere. I've heard that pressure should be around 30-40 lbs max, but don't take my word for it, find some old sheetmetal to practice on....


Moving on to the next phase of the test, the shrinking disc. The disc Mark sent me was a 5" diameter, and kinda gets in the way of using the 4-1/2" guard. To promote the retention of digits, I did install the side handle...



A quick check of our panel to find the high spot...





The panel was heated to a light brown color and cooled with a damp rag.



....and quickly found that the stud/nut of the grinder hangs too far down to use the disc flat to the panel.



So because the grinder is held to an angle to the panel to clear the stud/nut, the panel has a tendency to assume the rounded shape of the leading edge of the disc...



So right off the bat, my results will likely not be optimal, than if the spindle area of the disc were better offset to accommodate the spindle nut and permit the use of the flat area. Here is the next high area...



...and our cooling device..




Looking at the results of the shrinking disc, the use of the disc at an angle appears to have gotten rid of the high spots, but created other issues....



It's as if, in much the same fashion as the shrinks from the dent puller's heating tip, we would be better served by working this issue from the back side to better shrink the side that was damaged initially. Flipping the trunk lid over, here is what we see from the back side...





Since our tool has to be used at an angle due to the spindle, we should have clearance around the inner trunk lid structure. I'm not sure that a larger shrinking disc would permit this ability, but using the tool in the flat configuration would likely not cause the concave issue either. After using the disc from the inside of the trunk, here are the results....









The initial check on the outside shows some minor concave issues where we used the disc from that side...



Following the success I had with the hammer and shot bag on the other test area, I thought I'd use it here as well





In checking the area towards the forward edge of the trunk lid, a number 11 sweep seemed to match it pretty well, so we'll use that to check across the entire area...







Where it's certainly not perfect, it wouldn't take much more to get it to that "high build fix" stage.

 

Then, some days you can't see the forest for the trees.

A few days ago, Richard (TheRodDoc) on allmetalshaping told me he didn't think the outside was shrinking (being put in to compression) due to stretching of the other side. The measurements I was taking to hopefully show such a change seemed to support this theory, but he said it was a factor of the measuring process I was using. Now anyone who has seen Richards posts knows what awesome pictures he can come up with to get a point across. But he had not done so here (yet) and me being a visual guy, his message just wasn't sinking in. So I kept on measuring with my Pizza cutter until he'd had enough of my foolishness, and put up a picture I could comprehend.

Basically, anytime a panel is in the flat, you'll have indentical measurements on both sides. Introduce an oil can, and now the concave side is slightly shorter, the convex side slightly longer...


(photo by TheRodDoc)

Well this was like turning on a light bulb, and certainly seemed feasible. So I went back to all my pizza cutter measurement pictures, and all the measurements taken of concave panels seemed to show a shorter distance, while convex/repaired panels seemed to show the original measurement. But in all my pictures, I failed to take a measurement of both conditions after any one process of inflicting damage. So with 1/3 a bag of media left,


One more sandblasting evolution of the driver's repaired side...





Measured:





This "appears" to show a loss of length across the damage area.
Now, with some strategic placement of a folded shot bag for support underneath, and some questionable placement of ballast material on top to hold the trunk lid down and force the damage outward...(do not try this at home) .....with the sweep now showing a gap under the ends, our damage is now as concave as we're going to get....







And a quick check of the measurement across the same area shows the length has returned to where it started.





With Richard's theory validated, I must first thank him for his guidance, once I could understand the pictures, and for all you tool horders out there, I must apologize to those that rushed out to buy a new pizza cutter. Hopefully you still have it in the wrapper, and with Valentines day right around the corner....

The other suggestion Richard made was in order to fix it, treat it like a stretch, don't over think it.

Given all that, I think all that our pizza cutter measurements told us was which direction the oil can was facing, which was also plainly visible. Having used the three different methods, I think the shrinking disc was the quickest at showing results, but due to the fact we couldn't use it flat, it did cause other issues to address. But, this also allowed us to use it from the back side, where a flat only disc (or one of the larger ones) may have been a challenge. One of the larger discs used flat, may have been more suitable if you could hold the oil can in the outward position, like was show above. Steel toe shoes may be needed with all the ballast that was used.

Using the heating tip on the dent puller from the damaged side, IMO was very effective at returning the damage area to the original crown, with a small shot bag coming in handy for some of the "fine tuning". With as many shrinks as were needed, it is a slow, controlled process, and thus a bit easier than the shrinking disc in monitoring the progress. One would need a quality tool for as many shrinks as were used, or be sure to let it cool off once it gets good and warm. Where this process did show more promising results when used on the damaged side, Richard indicated he fixes this type of damage with a torch, which should give a more uniform shrink throughout the panel thickness. But if you have access to the backside with a sandblaster to damage something, then it should leave plenty of room for the dent puller and heating tip.

The donut dolly works well in that the nature of the operation is moving the panel back in the direction the panel belongs. This also is a slow, controlled process as compared to the disc, so it makes it easy to monitor the progress. This does require access to both sides of the panel, and is most effective with accurate alignment of the hammer strikes centered on the dolly. So if you see yourself having difficulty in this alignment, perhaps one of the other "one sided" repair apllications would work best for you. The only thing I think may have helped more in using this tool would have been to use the small shot bag instead of the hard dolly when I was attempting to planish remaining low spots.