Epoxy primers come in a variety of colors. Lighter colors are easier to cover with final topcoats, and produce brighter topcoats. A word about zinc chromate primers- zinc chromate has been used for years as a primer. Zinc chromate should not be used if you plan to apply polyurethane paint. The elapsed time between priming and applying the topcoat will vary depending upon the brand of paint used.
Different manufacturers recommend varying times.
As a general rule, an epoxy primer should completely cure and harden prior to applying the top coat. The curing process may take several days, depending upon the temperature and humidity. The primer should then be scuff sanded to obtain the needed adhesion for the topcoat. Proper sanding is a very important step in producing a high quality finish. If you want a smooth, high gloss finish you will spend time sanding.