One of the problem areas on any old bicycle is the inside of the fenders. These areas love to pick up junk, moisture, and eventually rust. Although collector covet all original paint, if you ride your bikes, you will want to protect the insides of the fenders in some way.
My approach is to rub in a matching paint. The amount of paint used depends on the extent of the rust and the current condition. Rusty fenders get more cleaning and more treatment than cleaner ones. The Schwinn New World had relatively minor rust, so I was able to use a thinned, matching automotive paint applied via a clean cloth. The paint was rubbed in. Using this method, the paint adheres to the bare spots but skims over the original paint.
The Raleigh Dawn's fenders are in worse condition. There was a fair bit of rust that I cleaned off using the hybrid method (see http://bikeshedva.blogspot.com/2012/10/removing-rust-from-fenders-hybrid-method.html ) and there was some pitting as well. In this case, I want to add a little bit thicker paint to cover the pits, get into the crevices, and treat the "rain gutter" in the center of the fender profile.
My approach was to take the matching fender paint I got this weekend (everyone loves the paint store shuffle) and to apply it to a painting sponge.
The result is that the fenders wells are rust free and green. I set them in the sun to dry, though this time of year the sun is a little anemic.
Painting in the winter is always a difficulty since it may not get warm enough to dry fully. I augmented the sunlight by moving them indoors and putting a space heater near them to help dry a bit. Remember, if you use a space heater don't put it near anything that can catch fire. There's always the story of the clown who put the space heater up against a bunch of paint thinner-soaked rags. Don't do that...
Here are the bike shed project bikes. From left to right: 1949-50 Columbia Three Star Deluxe (done), 1947 New World (almost done, needs saddle work), 1965 Raleigh Dawn Tourist roadster (needs work), 1974 Raleigh Sports (done).