Just a word of experiance. I would never use spray can undercoat for anything other than the wood maybe....great for protecting tree's after offing a limb.

It just traps moisture and flake's off. How do I know, cause i hate rust with a passion and have tried all sort's of ways to stop it on my truck. I tell ya after year's of restoring car's and trying all the fancy junk out there the thing that works best is a nice fat coat of zinc rich primer followed up with a nice cheap layer of oil/grease. I know it sounds kinda crazy but it works great. My favorite is a bug sprayer filled with WD40 thickened up with motor oil.