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Old 10-22-2014, 12:14 PM   #2
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Join Date: Jan 2013
Location: Leonardtown, MD
Posts: 1,290
Re: Repairing Sandblast Damage

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. This is an indicator of what I call a tight oil can, or one that has resulted from a stretch.

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, for a more effective off dolly process.

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.....

Last edited by MP&C; 10-22-2014 at 12:32 PM.
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