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Old 10-22-2014, 12:17 PM   #5
MP&C
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Re: Repairing Sandblast Damage

Moving on to the next phase of the test, the shrinking disc. The disc is 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.



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. Overall, IMO the disc works too quickly to be able to closely monitor the operation and keep results consistent throughout.


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, you can also fix this type of damage with a torch. But if you have access enough to the backside with a sandblaster to damage something, then it should also leave plenty of room for the dent puller's heating tip for repair.


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 applications 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.


Heating with an acetylene torch and shrinking the hot spot would be another option, but I'm a firm believer in using slower, controlled methods where you can "sneak up" on your expected result, and not blow past it. For those who are adept in using the torch, I would suggest small circles heated up to a dime size or less, cooled, and then move to the next area. I would also suggest practicing any of these on a scrap piece that has purposely stretched in a similar fashion, to prevent causing further damage to something you are trying to save. Once you get comfortable with the methods you plan to use, then jump in on the damaged "good" panel..
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Robert

Last edited by MP&C; 10-22-2014 at 12:25 PM.
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