Then again... if the varnish is sufficiently atomized, and at a high enough concentration, all it takes is a wayward spark. The sprayers I know who work in a shop-type enclosed space use explosion-proof fans. Why not use a water-based varnish and do a complete end-run around the problem? I've used a Thos. Moser water-based marine spar varnish with good results.Not only will it not blow up your shop, but multiple coats can be applied over the course of a single day.