Glad this came up again, I've been wanting to ask: Say you've got a half-full can of old varnish, stored under suboptimal conditions. You open it up, and there's that lovely rubbery skin floating on top. Why can't you just skim off what you can, and pour the rest through a funnel and coffee filter to remove any remaining nuts & bolts? The solids content would be only slightly decreased--basically like thinning. Wouldn't this filtrate be useful at least as a first coat, or in less critical areas, and preferable to waiting for a household hazardous waste dropoff event?