RepairNation.com - Cab Corner Repair Pg 2

Auto: Repairs: Autobody: Rust Repair - Pg. 2

Spray etching primer Step 6. Prime metal
One of the most discouraging problems when repairing a lower panel on a vehicle is rust reoccurring at the seam (moisture accumulates). When that happens, body filler can bubble and the repair looks awful.

Here's a trick to prevent it. Spray some 'Self-etching' primer along the repaired seam. After the primer has dried, grind or sand the metal surface again with course sand paper such as 40 grit. Most of the primer will be sanded away and the rough sand scratches will allow the filler to properly adhere. However, a small amount of primer will still be left in the seam where the sandpaper couldn't reach. It will prevent rust from occurring along the seam under the body filler.

Self-etching primer is a professional quality product that most body shops use to prevent metal from rusting. It's a fantastic product! It's rather expensive and far superior than normal primers you find in stores. Self-etching primer is now available in cans at most NAPA parts stores and other auto parts stores. It's definitely the product to use on bare metal.

Step 7. Apply filler
Now it's time to apply plastic autobody filler. Mix up a batch following the instructions on the can and apply it over the repaired area like shown. Make sure to apply over bare metal. The filler adheres to the sand scratches and pores in the metal. Never apply filler over paint. By using long even strokes like shown in the picture it can cut down on sanding time.

Make sure your metal surface is relatively flat and correctly contoured before applying any filler. That way a minimal amount of filler will be necessary and will help promote a longer lasting repair. When filler is applied too thick it tends to crack and eventually break leading to rust under the filler (filler absorbs water). 3/16" is about the limit. Apply filler in thin layers rather than heavy coats. Too much filler may require a significant amount of sanding -- and a lot of elbow grease.

Fillers come in varying qualities. High quality fillers available at most auto parts stores become tack-free in less time than standard fillers. That keeps sand paper from loading up with a tacky scum that often forms at the surface of applied filler. You can remove that scum and make your filler tack-free even more quickly if you wipe the filler with a paper towel lightly soaked with lacquer thinner after 10-15 min of drying time.

Step 8. Sand filler
After the filler has set up and become hard (about 12-15 minutes), sand it with course sandpaper (36-40 grit) and a sanding block. You can also use a piece of 8" wood as a sanding block. Don't wait too long to sand the filler -- it'll get rock hard and sanding it won't be fun. Sand the filler using long strokes along the length of the repair.

After you have the filler 'roughed out' switch to a finer grit of paper such as 80 grit. It usually takes more than one application to get it right. If so mix up another batch and repeat sanding process.

In the picture we are using an 8" gear-driven dual-action (DA) air sander, (better known as a "mud hog" amongst bodymen) to grind the filler smooth. A gear-drive sander is not the same as a regular DA sander, it's much more aggressive. If you plan to do a good deal of bodywork, it's a great investment. It makes short work of sanding filler (there's a reason they call it a mud hog).

Step 9. Apply more filler (if needed)
It generally takes a second coat of filler to fill pinholes and other imperfections in the filler -- even for experienced bodymen. After you've sanded the filler the first time, check for straightness using a ruler and apply a second coat of filler if necessary where needed.

Step 10. Final Sand
Here we are sanding the second coat of filler with a long, 17" sanding board. The long board knocks down any high spots and leaves a smooth, flat surface. Start with 40 grit and switch to 80 grit sandpaper. Use even finer sandpaper such as 120 after that. Primer will fill some scratches, but will not fill the deep scratches.

Final sand & prime

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