POR-15 is strengthened by exposure to moisture. Notice the hardness of the POR-15 coating. It doesn't chip, crack or peel like ordinary paints do, and its hardness will resist the wear and tear of everyday life. Rust conversion products claim to change the chemical nature of rust and convert it to a more stable element that won't rust again. History of the failure of these products is well documented, most simply don't work for more than a few months at best, and they are subject to the same chipping and cracking that occurs with ordinary rust coatings.
Most people brush it on, but many spray also.
Proper equipment and organic vapor particulate respirators for solvents are essential for spraying. You should use only POR-15 Solvent for thinning. Keep lacquer thinner handy for clean-up. The more humid the area, the faster the dry time, which usually varies from 3 to 6 hours. No, but if you do, remove it at once with solvent or lacquer thinner.