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Acid Rain: Sanding

January 22, 2014
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This is the sixth part in a seven-part series covering the repair process involved with acid rain and chemical etching.

For the purposes of this article, we will consider the following project: you are repairing a 1999 model vehicle. The paint has etch marks scattered across all of its horizontal surfaces, requiring that several large areas must be replaced. Please note that the items listed above (in particular, the paint thickness gauge), should be used in the following chemical etch repair process.

Sanding

First, it is recommended to check several spots on the vehicle to determine how much clear coat might have been removed from previous buffing or sanding. A typical measurement could show the total paint thickness from 4.1 mils to 3.9 mils. Since this measurement is above the 3.6 stop point, you can remove 3 mils of clear coat before clear coat repainting would be necessary.

You need to choose the correct grade of micro-fine sandpaper for the particular process you are using.

  • Light etching: A 2,000 grit sandpaper is adequate;

  • Medium etching: Begin with 1,500 grit and finish with 2,000; or

  • Heavy etching: Begin with a light 1,200 grit and move to finer grades of sandpaper, such as 1,500 and 2,000.

NOTE: Because different numbering systems are used to grade micro-fine sandpaper, be certain you use sandpapers graded according to American National Standards Institute (ANSI) standards. European grading, indicated by a “P” in front of the number, differs significantly from the ANSI system. For instance, their 1,200 paper is approximately equal to 600 grit by ANSI standards, a big difference.

To wet sand correctly, use micro-fine sandpaper that has been soaked in a solution of water and a few drops of liquid dish detergent. Then wrap the sandpaper around a semi-rigid sanding pad to evenly distribute the sanding pressure on the repair area. Using the pad helps you to avoid finger mark streaks caused when sanding by hand.

Sand using light, even pressure and work lengthwise with the panel. Also, keep the repair area flooded with soapy water. This helps hold the sanding debris in suspension and keeps it from further scratching the finish.

Stop often during the process and clear the area with a squeegee to check your progress. At this time also check the paint thickness to see where you are in relation to the 3.6 mil stop point. You stop sanding when there are no more signs of etch marks in the paint. The highs and lows in the clear coat should be completely evened out and the finish should be evenly dulled.

Check the thickness of the paint at this point. The total may now be down to 3.7 mils which is still above the 3.6 mil stop point.

NOTE: If you see any color on the buffing pad, sandpaper, or in the sanding water at any time during the repair process, this means you have broken through the clear coat and you have gone too far and the panel or area will have to be repainted.

Here are the other articles in this series:

5. Acid Rain: Buffing steps

4. Acid Rain: Preliminary repair steps

3. Acid rain and rotary buffers

2. What is a paint thickness gauge?

1. A car that's been hit by acid rain comes into your shop. Now what?

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