Underbody rust prevention

That’s what I’m figuring with my Subaru.

Sure it’s a '96, but it has a clean underbody right now, and only has 100k on the clock. If I can keep the body in one piece, it should last me a few years up in Vermont while I am finishing up school and then I can buy a newer car once I have a career.

So this stuff is getting put on each fall. When about is the best time to do it? Should I wait until November?