Minor body rust sanding and repair

Question

I had a minor accident a little over a year ago, the damage from which I did not immediately repair so a paint scratch has corroded over time. Finally, I'm getting the time to fix it.

My questions are:

1. How should I sand the area? I own a lot of tools, some of which are a 4.5" grinder and an orbital sander. Should I use one of them or something else? In case of the grinder, what grit flap disc should I use? Same applies in case the orbital sander should be used, what grit? Someone also mentioned I use a paint stripper similar to this:

2. Once sanded, should I apply a thin layer of body filler or should I just prime and paint over it? Is body filler used only in case of concave dents?

3. This question does not pertain to the picture attached. I have a variety of other small concave dings around the truck that have no rust. Should I just fill them with body filler over the existing paint and then prime and paint it or does the area need to have paint removed prior to.

4. Finally, any general advice in regard to making this and similar body repairs would be appreciated as well.

Answer 1

Bodywork can be really fun or it can make you cry.

There are some general rules to adhere to.

Rule 1 - Don't use anything over 400 grit on the plastic bodywork like the front bumper. It will scar it up.

Rule 2 - Put the power tools away when you are dealing with the flexible plastic, unless it's a sander (not a grinder) with 400 grit or finer SP.

Rule 3 - Don't put filler/putty on plastic. It's flexible and will eventually break free, bubble and look bad as it peels away from the plastic.

Rule 4 - 400 grit or less.

Some will argue with my rules, so be it.

Sand away the badness on the bumper (based upon the image) and get it to the plastic.

Get a paint mask. Protect yourself. The cartridge kind are the best. 3M makes good ones.

Use wet/dry sandpaper. The black kind. When your sanding, use water and the sandpaper. Water will carry away the waste.

Do a coat of primer. If you have scarring in the plastic you can use little tiny bits of putty but I've had putty fail me on plastic too many times. If you can prime/sand prime/sand prime/sand to fill, I've had much better results. I've learned though failure.

....and then do another coat of primer. Get it to be the way you like it. Sanded nice, no finger oil, no contaminates. Nice and clean. Use mineral spirits or acetone to wipe it down. (open air, fans, etc...while doing this).

Depending on the brand of paint you use you will hand sand the primer to either a 400 to 600 grit. The manufacturer of the paint should have a white paper detailing their requirements.

Once you've primered your very clean surface that is no more course than 400 grit you can lay your paint.

Layer paint is easier if you are going to follow the paint up with a clear coat. The clear coat allows you to overspray and compensate for your failures. Part of painting well is hiding your mistakes. Clear coat allows you to fail during the paint phase and compensate for all of the mistakes you make.

When you clear coat everything becomes shiny. If you clear coat a moderate amount, 3 to 4 sprays, you get to sand it down.

First using 600 then moving on to 1000 grit. You don't have to worry about the paint if you have excellent priming and sanding. Patience and cleanliness yield the best results. After paint, you get to sand the clear coat to 1000 grit and rub out anything you don't like.

Keep it clean, keep it flat. I can elaborate on any points. Best of luck.

Answer 2

The orbital sander would work fine. Use paint stripper only on metal body parts because it would dissolve plastic parts. Strip any area of paint that you intend to work on. Body filler won't hold well on a smooth paint surface.

Start with about 180 grit to strip the paint. Sand a reasonable area around the ding as well, 2 or 3 inches is fine. Make sure you dont have any harsh transitions from unsaded to sanded. Blend the sanded area so that it fades away as you move away from the area you intend to fix.

Use body filler to fill any concave dents. I recommend using the bondo brand. It goes on very smooth and is easy to apply. After you apply the body filler start sanding with increasingly finer grit, all the way up to at least 400. Apply a thin layer of glazing and spot puttly. Again, I recommend bondo. This fills any small imperfections, such as pinholes. Then do one final light sanding pass.

Body filler only has to be used for concave dents. If youre just fixing a scratch, sand it the same, and apply glazing and spot putty.

Then all thats left is prime and paint. When I was doing similar repairs on my front bumper, all I did was prep the surface in the manner I described above. I took my car to an actual paint shop to get the paint done. They can makes sure the colors match and that it's properly applied so your body doesnt rust.

Hope this helps! Best of luck.