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Acid Rain: Preliminary repair steps

January 08, 2014
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This is the fourth part in a six-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.

The vehicle should have a base-coat/clear coat finish, but if you are not sure of the type of paint system, there is a simple thing to do. Put a small amount of non-aggressive polish on a white towel and rub several areas of the paint. If no color transfers to the towel, the vehicle has a clear coat finish. (If color does transfer to the towel, you are working with a single stage paint and the methods described in this article may not work for this particular paint system.)

Once you have established the paint system, choose a test area of about two-feet by two-feet on which to see whether the damage can be repaired without repainting. The test area should representative of the overall severity of etching damage on the finish. It should contain some of the worst and some of the least severe etch marks.

If it is impossible to find a two-feet by two-feet area like this, test the procedure on the least severe spots first. If the damage is scattered all over the horizontal panels on the car and these least severe spots cannot be repaired without disturbing the basecoat, it is time to forget this type of repair and tell the customer the vehicle should be repainted.

With a test area chosen, you need to know whether a paint sealant has been applied to the finish. If it has, the sealant must be removed with a solvent. If it is not removed, the sealant may slow down the process and end up being ground into the finish.

Any paint finish without a sealant application requires only a good washing with a mild detergent to neutralize and remove contaminants and dirt, and a thorough rinsing. Of course, stubborn substances like wax, tar, bugs and grease must be removed with a solvent before washing and rinsing.

Next, measure the total thickness of the paint in the test area. This allows you to know how much clear coat you are removing during the repair process. It also allows you to calculate a “stop point” beyond which you can go.

OEM factory applied clear coat is usually 1.5 mils thick. It is safest to leave at least 1 mil of clear coat to protect the basecoat. So, no more than .05 mils should be removed from the total film thickness. If more than that is removed and the base-coat is not disturbed, the clear coat must be reapplied on the repair area to restore proper base-coat protection. In addition, it becomes necessary to reapply clear coat, the last thickness reading can be compared to a reading after the clear coat has dried. This allows you to determine whether adequate protection has been restored.

For example, the paint thickness in the test area may be 4.1 mils, so it would be safe to remove .05 mils. Your stop point is 3.6 mils. Reducing the total thickness anymore than this will remove too much of the clear coat. As necessary, you can buff or sand until you reach 3.6 mils. Go beyond this point and you may need to refinish, at least with clear coat, and if you break through the base-coat a refinish for sure.

Be sure to make it policy to measure the paint thickness on every vehicle which you are trying to remove chemical etch marks. Different vehicles have different total film thicknesses. So, the stop point for each vehicle will be different. Do not assume that the starting thickness is always 4.1 mils and the stop point is 3.6, that is not true.

Here are the other articles in this series:

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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