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Sanding and Finishing Metal

By, Matt Joseph

This tech tip is from the book, Automotive Bodywork and Rust Repair.

The repair of two small dents in a 1948 Chrysler decklid illustrates the basic sanding-picking-filing-sanding sequence that is used in metal finishing. This process indicates low spots, grinds off high spots, and ensures crown continuity in panels. It is also used to raise and repair small dents. 

The first step was to disc sand the paint off the panel, in the area of the two small dents that are the subjects of this repair. Since the rest of the panel was undamaged, and because our plan called for chemically stripping the panel after the dents were repaired, we disc sanded it only in an area that included the two spots. This would allow us to check crown continuity in that area, after we made necessary repairs. 

sandingpaintoff                     

1. Sanding the paint off this decklid revealed the extent of the two medium-sized dents that were the reason for the repair. Both were perfect candidates for hammering up from the other side, and metal finishing, because both offered good access for that procedure.

The sanded area revealed the extent of each dent. It was about what we expected, and well within the range of what it is possible to repair by lifting up and filing metal. Our first step was to pick up the smaller spot with a fairly blunt pick hammer. The larger spot was first driven up with a small, highly crowned body hammer, which reduced its size by more than half of its diameter. Then, we used a blunt pick hammer to finish lifting its center.

 

 

bluntpick                 

2. The smaller dent was driven up with a blunt pick to raise its deepest area and, at the same time, to slightly raise the entire area around it. Picking has to be done with the intent to completely level entire damage spots.

At this point, the panel was filed with a flat 8-tooth-per-inch body file, held in a slight curvature in a flexible file holder. The first hammering and picking operation moved most of the metal to level. A little more picking and filing followed. This revealed that the damage was completely removed, and that the panel crown around and between the two dents was consistent and correct.

 

 

sandedbare                 

3. The areas around and between the dents were sanded to bare metal, so that the repair could account for the crown in the entire repair area. The dent on the left is more complex than the one on the right.

 

 

dollythatwrapped                  

4. We used a dolly that wrapped around two sides of the dent to back up our hammering. This controlled the movement of the panel area just adjacent to the dent. Hammering was done off-dolly, and the dolly was shifted from time to time around the dent.

 

 

filedwithflatbodyfile               

5. After the first picking operations, the repair area was filed with a flat body file mounted in a flexible holder. Most of the metal came level at this point, but some additional picking and re-filing were required to completely remove the dents, and to give the panel a consistent contour.

It remained to disc sand the entire area around and between the dents with an 80-grit disc, and then to scuff the area with an 80-grit DA random-orbital sander. This achieved a surface that was smooth, and that had good tooth for primer adhesion.

 

 

smootheddisksander               

6. The repair area was now smoothed with a disc sander, loaded with 80-grit paper. This removed most of the scratches resulting from the paint removal and filing operations. Note the almost flat position of the sanding disc for this procedure.

 

 

filedpanel               

7. The filed panel looked like this. Note the filing scratches in the repair area. This repair required so little filing that there was question whether  the repaired panel had any badly thinned areas or spots.

 

 

afterscuff               

8. After a scuff sanding with 80-grit paper in a dual action (DA) orbital sander, the repaired surface was smooth and had good tooth to hold primer. The completed repair is shown here.

 

If you like this tech tip, you will love the full Automotive Bodwork and Rust Repair book!

 


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