Paint Hardener

If you are going to use a hardener, I recommend using it in ALL coats unless the manufacturer specifically states otherwise on the technical data sheet for the exact product you are using. I am not familiar with the NAPA product line (Martin Senour) but have never seen in PPG or DuPont any reference to using hardener in only the final coat. I am not sure why you would want to limit it to just that. If you don't use a hardener in the first coats, you may have a problem with proper curing if it is a urethane or a problem with wrinkling (lifting) if it is an enamel when you go to apply the final coat. Do it right and follow the manufacturer's specific recommendations. That's the very best way to avoid problems. You should be able to get the tech data sheets from your supplier or on line. Do a search on "Martin Senour". The tech data sheets contain more usage instructions than the can labels do.

For hardeners, don't forget the proper safe breathing apparatus. Read the fine print on the hardener label and follow it. It's not just a CYA thing. It's there for a reason.

Rod