I have an older skillet that Was used for an ash tray for about 30-40 years and I can't get it to stop turning cleaning water light rusty color. Any ideas? 
I'd suggest, firstly, scrub past the rust on the surface using iron wool. Secondly, soak the skillet in pure distilled white vinegar for 6-12 hours. Don't soak it any longer as it could erode cast iron. Once you are done, season the skillet immediately to avoid further rusting on the surface. Hope it helps! 