If you want to retain the brushed look, have the frame clear powdercoated. But clean it well with wax&grease remover first. For extra durability, paint a top quality clear over the over the powder coat. Be sure to clean the frame w/wax & grease remover, then just spray something like PPG DCU 2042 (2 part) clear. Put about 3 coats and you'll be good for life.
The reason to paint over powder is that powdercoating will adhear to the metal much better than just clear paint. So why spray clear paint? Because when the powdercoat is baked, surface tension causes the melted powder to pull back from sharp edges. (especially the edges of cable guides) This is where the rust will start. So, spraying with hi-tech 2 part clear, the clear will adhear to the powdercoat very well and seal all those edges. Plus the clear paint can be VERY glossy, unlike the somewhat duller finish of powdercoating. Also, if you pwdercoated with metallic or pearl color, the metallic or pearl is on top and will dull or disappear after a few cleanings and where the cable housing rubs.
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Ciao 4 Now,
Dr.D