Anything that would hold moisture, such as kitty litter, is actually working against you in this instance. Treat the area with a rust converter or neutralizer prior to a good coat of paint. Lots of great epoxy paint products are out there for covering rusted areas (POR 15 is one), some are available locally while some are interweb only. For unseen areas I've had decent luck with naval jelly followed by a good quality spray paint.
http://www.loctiteproducts.com/p/s_t...-dissolver.htm
Great progress!