Painting them is nice if there is no great value. Of course if you have a rare one then do not paint them. Collectors don't buy re-painted toys. Part of the desire to them is the wear or patina. I don't collect them but that is what I saw on the Antiques Road Show.
IMO old cars are built pretty tough to keep up with kids. This is one I found 2 years ago and had in a box of other toys. When your question inspired me. In the beginning it had no rust pretty dirty and had some of the paint left. I cleaned it the best I could by hand removed the tires, cleaned them with dawn and a sponge then I used the metal brush and my dremel on setting 5 I cleaned off the excess dirt and paint. I then put two coats on and one coat of a low gloss finish here is the result.
