Here are a couple of methods I've had success with on my models:
Inkjet prints can't accurately represent polished bare metal, such as wheel rims and suspension parts on our Indy cars. Such areas are printed in a light gray color. A much better looking solution is to color over those areas with a silver Sharpie marker. Use a draftsman's circle template on the outer wheel rims to keep the colored edge neat.
Always touch up edges of colored areas so the white edge of the card doesn't show. Kid's felt tip markers work well for touching up reds, yellows, and dark colors (but note warning below). Colored pencils do well with all other colors. Best of all are watercolor pencils, dampened slightly before applying. Whatever method you use, test it on the edge of a scrap piece of card first.
If you are going to use the clear-coating method below, it's best not to use markers for edge coloring. The solvents in the wood hardener and paint will dissolve the ink and allow it to seep through the part. If using silver Sharpie on parts, wait until after clear-coating to attach or color them.
The method I use to clear-coat models is to first apply 3-4 coats of Minwax Wood Hardener. Allow to dry overnight. Then apply 3-4 coats of Krylon gloss clear spray, allowing to dry between coats. The reason for the wood hardener is to both harden the paper, making the model more durable, and to seal the paper so the clear doesn't soak in, reducing the number of coats needed. If you don't use the wood hardener, be prepared to spray at least 10 coats to achieve a decent finish.
Created on ... January 23, 2004