I use pine varnish, hard, brittle pitch dissolved in alcohol over sinew wraps on arrows but not on bows. I guess if you made true varnish with a solvent, an oil and the pitch it could be done. The oil would soften it up enough to allow constant bending, make it more flexible. I think hand rubbed fat(bear, deer, etc.) heated and mixed with pitch would be a better option but it would only be water repellent and not water proof.