Grind it down, you can use a grinder or an 80 grit disk on a drill, can get a disk kit for a drill at Home Depot. Dremel tool would work good too because its a small area. Grinding it down is no big deal, its already screwed up (in case that freaks you out), Be careful not to grind a whole through the sheet metal, just lightly grind the infected spot until the main crusty rust is gone. Hopefully its just a rock chip that got a little salt on it. It looks like the lower one is in a spot where there may be a spot weld/seam, two pieces of metal close to each other. It will most likely be hard to get to all of it if its spread inside there. I second the Por-15, that stuff is really good, I have used it many times. They sell a repair kit that has an acid that eats the rust and kills it in the kit, in addition to the paint.
www.por15.com/SUPER-STARTER-KIT--Black_p_11.html I believe its black paint only so you could paint white on top of the black por-15 paint once done. Naval jelly is the same, its acid, you brush it on thick, let it sit a couple hours and hose it off, then do it again, and again until the rust pits will scrape off or until they are gone. Its acid, so wear goggles at least, water deactivates it. Blow dry it with air once you hose it off. Don't worry about the water rusting it again, blow dry it. Its the salt thats the problem anyway, not really water. Don't use that rustoleum junk for paint, or any other "rust paint" it won't work, you have to grind out rust and kill it first, period. Sorry, I have restored classic VW's for the last 20 years so this stuff gets me worked up. Hope that helps.
