you can buy rust removers which you paint on and wash off...... its basicly an acid!! like paint stripper but strips rust! halfords sell the stuff.... jenolite
main thing is if you cant remove it all get it as dry as possible for painting over it! it also depends on what type of rust it is as some pits the metal! so it all depends on wether its surface or its penatrated deeper!
also you can buy rust converters... like krust..