I was reading a woodworking magazine and it had an article on coatings. One section was on varnishes and it explained what the types were. I started googling varnish, mostly for WW1 wood frame models for my use. Probably what was on the planes was a derivative of what they call "boat soup". It would be cheap to recreate but probably not a good thing for models. Violin varnish looked like the perfect thing, but the real stuff is expensive, like $200+ an oz! Then I came upon this and it looks promising; looks like it can be mixed with just about any carrier. http://www.stewmac.com/shopby/product/5032