A good choice is Oil Eater, use it full strength; it will soften paint, but it will re harden. I would definitely not use volatile flammable solvents, perhaps kerosene or paint thinner might be OK, but Oil Eater is a better solvent than either one of them; life is too short for serious burns; do not get me wrong, I use acetone, MEK, lacquer thinner and suchlike for small cleanup jobs, but not big cleanup jobs like machinery.
For machine ways with surface rust, I first use a carbide scraper, then go to an india oilstone and liberal solvent to keep it from getting clogged up. Richard King uses this stone method with windex for deburring scraping work.