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Before blasting.... (archive)

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Posted by kwillmorth on August 03, 2001 at 09:38:17:

In Reply to: Performance BMW to repaint wheels (more) posted by Dino on August 02, 2001 at 23:38:28:

Sand blasting cast aluminum, particularly permanent mold and die cast parts (used for quality wheel production), is not a good way to strip them. Professional wheel refinishers (like Prestige in LA, who specializes in Porsche wheel refinishing), use the following process:

1. Chemical strip of old finish
2. Media (plastic or baking soda) blasting of stubborn finish in corners, lug bolt holes, etc. (leaves no surface texture).
3. Surface "polished" using 3M abrasive pads, or 600 grit sanding media.
4. Repair surface damage as required, machine and etch repair areas.
5. Chemical cleaning and etching.
6. Chemical chromate conversion coating (corrosion inhibitor and adds grip to the overcoat finishes)
7. Epoxy or catalized Primer undercoat
8. Finish color coat(s)
9. Clear coat(s)

This produces a finish that is as good as or slightly better than the factory finish.

Sand blasting a cast part breaks the as-cast surface of the metal and produces a texture, it also opens the surface "pores". Inside many of those little pores are flecks of iron from the casting tool, which, when exposed, will cause corrosion under the paint (called filiform corrosion, evident by little blisters that grow and push the paint off in patches). It works fine for an engine block, but is not something you want to do before putting on a high gloss, high quality paint finish. Sand blasting will also leave a noticable texture, that will be seen through the finish coat.

In painting (we do tons of aluminum finsihing every day, literally), the most important step is the surface preparation. Sand blasting is fast and easy, but an inferior surface preparation. The finish will have an undersirable texture, and will not be durable.

Another alternative is to just sand the old finish smooth with 400 to 600 grit paper, use an epoxy filler in the deeper nicks, and and scratches. Then use an etching, catylized primer over the old paint. Over that you would do the same base/clear over coat. This keeps the surface smooth, and will adhere at least as well as the stock finish sticks to the wheel, which is not a problem in general. I had this done on a set of Jaguar XJS rims that saw Minnesota winters on a daily driver, and was very happy with the results.

For chrome preparation, blasting is an absolute no-no. You'd have to follow that up with some serious polishing to get the surface ready for plating. Plating requires a polished substrate. This can be done chemically on an as-cast surface, or mechanically on a sand cast or blasted surface.

Chrome versions of the roadstars can be had from most BMW dealerships on an exchange basis, for around $700 a set. They use a chemical strip, chemical polish preparation process, which is quick and easy. Might be a better alternative than a poor repaint process. Cheaper for the dealer I would guess as well.

As far as the color of the rims are concerned. A good paint supplier can match the color with some experimentation. It is a three part process, starting with a pale grey base, and a semi-transparent metalic overcoat, then a clear top coat (which the stock wheels seem to not have BTW). I've seen some near-chrome finishes devised that are quite spectacular.

Anyway, my twelve cents worth for the day.

Everyone with fading wheels should get the fixed for FREE, but I have a question below.

Performance BMW in Chapel Hill, NC, has agreed that my wheels are fading, and is willing to sandblast, and then repaint and finish them. The even said that they do the clear-coat, but in opinion, there does not appear to be a clear coat on the original finish. Hell, there is hardly even a silver coat .

QUESTION. I have a few tiny scratches in my wheels. One chunk from a torque wrench. Two nicks from a clamp that Firestone used. A tiny nick from BMWs tire mounting machine, and some minor rash on one wheel. Since I'm getting the wheels re-painted, should I have these things repaired, or is the sand-blasting and two-coat paint job going to cover it all up anyway? Does anyone know if the tires need to be removed?

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