Not sure what you mean by painted on markings. I’m guessing your talking lettering. The trouble with anything liquid type it also removes the paint underneath or at least it softens it up. You could consider a really fine grit sandpaper. Dip it in some water or sand it in the sink.
If it’s just lettering. I’ve been recently changing rd. numbers on rolling stock in order to build up a fleet of hoppers. I bought a Paasche Air Eraser and just mask off one or two numbers. It removes the numbers easily and more or less just dulls the underlying paint. A coat of clear gloss and add decals with different numbers and it’s done. I wouldn’t advise trying to strip an entire car with this method.