Talus

I've got enamel touch up paint to cover some nicks and rusty spots on a Centurian frame. I just can't get the stuff to mix evenly. The metallic flake is darker than the paint itself and just brushes on unevenly. This isn't that noticeable on small dings, but the
larger spots really look crappy. Any ideas?

Captain Blight

Try mixing in a little white enamel to lighten it up, use some thinner to get it to flow better, and feather out the edges so you get an irregular fade out to the rest of the paint instead of a clearly-defined demarcation line.

gaucho777 

Touching up metallic paint can be a challenge. If you haven't already done so, you can wet-sand the area your've touched up, though you are not likely to make it perfectly smooth at this point. Not sure how much work you are willing to do or how much you are willing to spend. You might get better results if you are able to get the paint in an aerosol can. Most auto paint stores can match the paint and put it in spray cans for you--but you'll probably spend ~$15 for the spray can versus ~$3 for a small bottle of touch up paint. (Often times the metallic hues are made with a 2-color process requiring a base and top coat; at least, that's was the case with my pearl white Look.) For the larger touch up areas, you may get better results by sanding off the uneven touch up you did to about the size a half inch circle, mask off that area, and then spray on the touch up paint to match the surrounding color.

dddd

I agree that the paint probably needs a bit of the correct(???) solvent added, and note that the particles begin to settle unevenly as soon as the wet paint is allowed to sit. You might try situating the actual small surface more horizontally to better defy gravity, and apply thinner layers which dry faster.

But the most important thing to me when touching up an old frame is to minimize the area that gets touched up. I simply fill in the defects with the best color match I can come up with, making very sure not to overlap the surrounding good paint (which thus will expand the touch-up zone, and which always make a touch-up stand out badly).
For this reason, I almost never sand down a paint-chipped area, but rather I work completely within the confines of the original defect.
Also, with darker colors, I always err on the side of a too-dark touch-up color mix, and usually apply the paint only with the tip of a toothpick.

I am talking here about the smaller touch-ups, not large areas of course, and I should mention that the air is dry enough here that I don't have to worry about using primer or sanding down to bare metal.