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Old 10-22-2014, 12:15 PM   #3
MP&C
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Join Date: Jan 2013
Location: Leonardtown, MD
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Re: Repairing Sandblast Damage

I had received various comments and feedback from the original post, so here's a review 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...

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I think the stretching caused by sandblasting is unique, and not like that caused by using a hammer and dolly. The metal isn't affected all the way through the panel, only one side. So if you could shrink that one side somehow, I can see how shrinking would work. But IMO you need to get the entire area of the panel stress-free or uniformly stressed and ready to shrink.

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. 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?

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How about sandblasting it on the outside to stretch the metal evenly on both sides? It will probably oilcan and need shrinking after that, but at least it wouldn't be stretched on only one side and you could use the shrinking disc to bring it back down. I still think a hammer and dolly would work too...that's what I would try anyhow.

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.


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Keep in mind that the shrinking disc works best when you lay the disc on the center of a high spot. I have not found it to be very useful when using the edge on a low spot.

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....

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How about sandblasting it on the outside to stretch the metal evenly on both sides? It will probably oilcan and need shrinking after that, but at least it wouldn't be stretched on only one side and you could use the shrinking disc to bring it back down. I still think a hammer and dolly would work too...that's what I would try anyhow..............I believe the right thing to do is to stretch the outside of the sheet while pushing up on the dolly, which is held on the inside. I think that action will tend to compress the inside (or stretch it less) and stretch the outside, resulting in a more evenly stretched area. Then you can shrink the whole area back to where it belongs with a shrinking disc. That's my thinking anyhow.........
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..


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if I hold the panel and use the propane torch method how long should I apply heat? Also should I heat just the center or the complete warped area?


Not long at all, very short, you can see the metal move. Too little is better than too much.
I moved the torch around in a circular pattern over the area for just a few seconds.
I did enough just so the panel would stay in the out position on it's own. Then I used the shrinking disc from there. Used the slapper and dolly if I got too much shrinkage.
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..

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You can get lucky on a few small pieces but when a large panel is damaged that bad you can chase an oil can all day.
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..

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Having absolutely no knowledge in body work probably tints my comments, but here goes.... Wouldn't it be cheaper to buy another trunk lid? Seems like a lot of man hours into R&D to save a trunk lid that can't cost more than $500 to replace.
If accurate replacement parts are available for said vehicle, then it is a viable consideration for the owner. If the parts are inaccurate, as is many times the case, many man hours will be expended to use those parts but make them and/or the adjoining panels suitable (think gap joints, etc). If someone is tackling sand blast damage repairs themselves, then it would be their time and effort. If parts are not available, then you either need to reproduce that part or fix what you have. I am conducting this test in hopes of finding the most effective method(s) of repair so that the owner of the car in FL can determine if this is a repair he can do on his own, whether it is one he should farm out, or as you suggest, buy a replacement.

We've got a shrinking disc on the way, to use on a test of 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.
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Last edited by MP&C; 10-22-2014 at 12:37 PM.
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