The standard powder solvent cleaner for ages has been Hoppes #9. If you have rusty spots on the exterior (barrel, receiver, etc) Go to the grocery store, housewares aisle. Buy a COPPER Chore-Boy pot scrubber. Wet the rusty area with a light oil, such as Remoil. Let soak for a few hours. Wet the Chore-boy with some additional oil, rub gently until rusty area is cleaned. Gun will take on a coppery color. Use a clean cotton cloth wet with a little more oil, rub- copper color comes off.
Routine maintenance? http://www.marlinfirearms.com/pdfs/manuals/MFC_Centerfire.pdf