Originally Posted by
OlongJohnson
The 642's finish is anodizing, which is basically a honeycomb-like lattice of aluminum oxide grown on the surface of the base metal. If the anodizing is colored, these cells are filled with dye. The "sealing" step is where the tops of the cells are closed over, making the surface non-porous and locking in any dye. The oxide is very hard (aluminum oxide is used to make sandpaper), but it's very thin. It has good abrasion and wear resistance and, AFAIK, will withstand any common hydrocarbon. However, it is extremely susceptible to chemical attack by acids and bases. The total anodizing layer thickness is usually on the order of 0.001", with the "sealing" portion where the cells are closed being a small fraction of that. So it takes very little to open the cells. Once opened, the dye, if present, can be substantially removed, so you get streaks in the color, and the corrosion resistance is diminished. It is also susceptible to staining due to absorbing new materials, such as inks or nail polish diluted with solvent.
Some caution regarding what gun cleaning chemicals are used on anodized surfaces is wise, as many of them may be acidic or basic. (I can't even remember how many incidents I've dealt with of "professional" car detailers causing cosmetic damage to expensive wheel and brake components by spraying them with acidic wheel cleaners. Let's just say that complimentary wash at the BMW dealer may not be your friend.)
AFAIK, nail polish remover is primarily acetone, so you shouldn't have any direct worries. If you want to be safer, you could buy a small can of just acetone at a hardware store to ensure there are no added surprises. However, if the surface of the anodizing wasn't perfectly sealed in production, or has been compromised at any point, it could absorb streaks of the color. So some care is warranted. It might be a good call to put down a layer of clear nail polish before any color, just to be sure that any material that's absorbed doesn't stain the finish. Wiping down the area with penetrating oil awhile before using the solvent would be another layer of insurance, as it would tend to fill any pores and prevent new materials from wicking in.