You can remove some of that left on color with a rag with some acetone on it then a little sanding. I spray down the air frame with the compressor then a tack cloth. That's about it before covering. If for some reason you want your covering to really stick to the wood you can spray the frame with lacquer hair spray. {easy} There really is no reason to do that though. It really makes recovering a harder job when it's time to pull the old covering.