In "Model Airplane News" a product called "Dawn Power Dissolver" was reviewed and a member of a local club tried it and said it works. I have not tried it so I am only repeating what I heard. He says all he did was spray it on, wait a little bit, then brushed the engine with an old toothbrush and the varnish came right off. He tested it on a 20+ year old Fox .35 engine and the picture he showed makes the engine look like new - all bright and shiny...