How do you properly and economically treat a blister
 

Hi there,
I have a blister on the rear hatch of a 1990 300TE Smoke Silver. Upon close inspection, it looks to be a bit rusty as well.

This blister is about 1 inch in diameter, and is in a REALLY noticable spot. What is the normal procedure short of painting the whole hatch?

Has anyone ever used a spot blaster? I have an aerospace quality spotblaster that has nozzles that allow you to blast a spot as small as 1/2 inch without blasting anymore than that. If I blast off this blister, it will basically feather itself. If I then use MB touch up and primer...would that be about as good as it gets?

Thanks

I'd say yes, that would be the way to go. I have one of those spot blasters too, and they are perfect for a job like this. The trick will be to minimize the area that you'll need to prime and paint. These little spots get bigger as you progress with the sanding and blending.
I'd use a bit of finishing filler, like Featherfill, if there are severe rust pits after blasting away the rot. If not, just prime, sand, color coat, sand, clearcoat, and final sand to blend. I just know you won't get it to be invisible, but a good job will not stick out like the sore thumb you have now.

Sometimes it's possible to repair and paint only part of a panel, but there needs to be some sort of parting line, like where the sheet metal is creased along the width, in your picture, just above the chrome strip. I don't know about the edges, whether that ridge goes all the way across or peters out toward the outside edges of the hatch.

thanks....I will get my ducks in a row before starting this. I will post my results...probably about a month from now. Gotta put some time into another car that is having a problem... :rolleyes:

I recommend that you have a high end bodyshop that specializes in MB and BMW paint work take a look at your blister. Better to have it professionally repaired than make matters worse or hide a real problem. I expect that they've BTDT.

After a straight week of rain, I noticed very small blister similar to that, where the front of the rear bumper meets the fender.

I peeled back the trunk liner to check for inside corrosion, and there is none. Appears to be just some paint flaking and light rust due to water accumulating at that spot, where the bumper and fender make contact.

I plan to scrape away the loose paint, sand the exposed area, and apply the Mercedes touch-up paint. The body color is 040 Black, which I understand is not clear coated. Should I be using a primer/filler on such a small area (a half-moon shape, approx the size of half a nickel)?

I made this discovery while examining some abrasions on my rear bumper, after I scraped a dumpster while parking. My second question is: Is blending simply a matter of painting-sanding-clearcoating-sanding and then clearcoatiing again? I ask because the Mercedes bumper touchup paint appears to be wee bit darker than some parts of the bumper, depending on the lighting and angle of the can when comparing it to the bumper. But I am attributing that to optical effects, as well as slight fading of some bumper surfaces over the years. I assume there will be a perfect match after spraying, and certainly after blending.

Whether you'll need filler or not depends on the penetration of the rust, under the paint/primer. If it has etched small pockmarks in the surface, which will be apparant after the rust has been removed, you might need a skim coat of filler. Often, a thick primer will do this for you. I avoid spray bombs of primer if that is the case (way too thin, which it has to be to go through the nozzle). I use a urethane primer-surfacer, mixed to manufacturers spec, and sometimes a wee less reducer to thicken it up.
As to the blending process, you need to think of the surface of the damaged area, after final sanding of your primer and/or filler, as being just below the surrounding area by the thickness of the finish paint coat(s). If you are using straight enamel (sprayed from a bomb or paint gun), just build the repair surface up to just above the surrounding paint, then sand it down until you are level (I use lots of 2000 grit wet). You'll see the blend when the edge of the area around the spot begins to get removed by the sanding.
If you are using base/clearcoat, color fill to just cover the primer with enough to blend with sanding, then clear and sand level like enamel.

Both paint jobs went well.

After some sanding and primer, I was able to brush the touch-up paint onto the fender, which I will sand smooth after the paint fully cures in 24 hrs or so.

The bumper was easy. I was able to sand away all of the scratches, except for the longest, deepest one, with 400-grit sandpaper. The bumper spray from the dealer ($40.00 :eek:) matched perfectly, and after applying clearcoat I could not differentiate the new color from the existing, so no blending was necessary.

Thanks for the good feedback.