I disagree with any method that uses spot putty (or other filler) to fill in scratches - here's why: If you fill the scratch with a filler prior to the paint the your finished repair will be higher than the surrounding area. Even if the filler shrinks prior to top-coating it will typically "dirty" up the edge of the repair and then you end up with a ring around the edge of the repair if you try to sand it flat.
Instead I like to use the color simply for that - color. Then finish filling the defect with clear urethane which can be sanded and polished flat for a nearly undetectable repair.
Here's the process I use...
Paint Chip Touch-Up
I'm an old body shop and paint guy. Don't do it any more but did a lot of custom cars and bikes back in the 1970 – 2000 time frame.
The best process I've found for touching up chips on BC/CC finishes (short of repainting the entire panel) is this...
1) Using the edge of a razor blade or Exacto knife, test the margins of the chip for adhesion and carefully chip/trim away any paint that is not firmly adhered to the substrate.
2) Clean the damaged area using a swap and solvent. Enamel reducer or alcohol is my solvent of choice as it will not damage the existing painted surface.
3) Using a fine tip artist’s brush, dab the chip with the appropriate color. Do not try to make the color coat flush to the existing painted surface - you need room for the clear-coat. Minimize any application of paint outside the margin of the chip being repaired. If you do exceed the margin, clean away the excess as described below in step 6). As with most paint applications, multiple thin coats/layers are preferred over a single heavy coat.
4) Allow the color coat to dry thoroughly. It will shrink a little as it dries and if you did step 3 correctly the touched-up color surface will be lower than the existing paint surface.
5) Carefully clean away any remaining excess color that may have exceeded the margin of the chip. If you're really careful a new and sharp single edge razor blade works well. Very fine (i.e., 1500G) wet-or-dry sandpaper is another option.
6) Again, using your artists brush, apply the clear coat in thin coats until the chip is filled flush or slightly higher than the existing paint surface. As in step 3), minimize exceeding the margins of the chip. Where you do exceed the margins or your paint build is too high, squeegee or slice away the excess using the edge of a credit card, a single edge razor blade, or a swab dampened with an appropriate solvent. Any smeared paint resulting from squeegeeing can also be removed with a swab dampened with an appropriate solvent, or with a light sanding once dried. Slightly exceeding the margins with the final coat is acceptable, and even desirable, as long as it’s a very thin coat. Allow to dry thoroughly between coats and overnight or longer for the final coat.
7) Wet sand the repaired area with 1500G wet-or-dry paper.
8) Polish the repaired area.
It takes some time and patience, but done correctly, this process will yield a virtually undetectable repair on solid colors such as black, white, red, etc. Other colors such as metallics, pearls, etc., are more difficult because it's very difficult to match the lay of the metallic when touching-up with a brush.
All of the materials needed including the paint can be obtained at most automotive paint supply stores.
Hope this might help some of you in the future.
As for the original question about thickening the touch-up paint I agree with the previous posts - just pour out a small amount and let it air dry for a while.
