Spraying Drylok Paint

You'll have some waste with spraying. Considerably more than with a brush or roller, anyhow. Find the square footage---both sides---and divide 325 into it. You'll get about that amount, 325 square feet per gallon, if you're careful. You will likely need more than one coat…

Also, make sure the surface has been well prepared by removing loose paint and sanding rust spots. You'll want a good primer, preferably oil-based, on those bare spots.

Sand evenly across the entire board to make it smooth. Clean and remove all dust from sanding. Respray as evenly as possible. If you can get away with doing it indoors (a garage for example) that is recommended to reduce the number of airborne particles that land in the paint before it can dry.

To ensure an even coat my personal method is to gauge the coverage by sheen rather than color. Meaning that if you view it at an angle the wet portions will be shinier than the less covered portions. Color can be deceiving, look at the sheen, if the sheen matches (as you spray) across the piece, the final product will be more even as well.

Keep in mind that with spray paint it takes less time to do extra coats of paint than it does to put on too much and fix a poor result...tempting though it may be to try.