Try wiping the base off with rubbing alcohol on a rag before advancing to lacquer thinner. If you didn't activate the Chromabase and just reduced it, this ought to work. If what I think I read is correct, you'll have to step up to lacquer thinner but try rubbing alcohol first since it is way less aggressive and cheaper. The alcohol(like you can buy in any pharmacy off the shelf) will remove uncatalyzed base and clear, I just learned that trick due to an airbrush artist friend of mine having a learning experience on my son's hood. Talk to the store mangler at Slo Reilly and ask him if there's anything he can do for you, they'll usually jump through hoops to please you if you're a regular customer and courteous. Our local Slo'Reilly mixes nason, Chromabase and Spies-Hecker. Chromabase is a premium base and in 20 years of spraying as a hobbyist, I've not had bad luck with it. As for sanding, wet sand it to no finer than 600 grit so the new base has something to bite to.