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Ask a pro: Tung oil protection
I am going to be using tung oil as a finish for my current project. I would like to complete it with some sort of protective layer, but I'm not sure what would work best with the oil. Does using tung oil limit my choices? - Jimmy See, Kingston, Ont.
I’m not a fan of putting a hard finish over a softer one because the top layer can crack as the wood expands and contracts. If you do wish to use a more protective finish on top, I would stick to an oil-based alkyd or polyurethane varnish. But keep the tung oil to just one coat so that it doesn’t build any significant thickness underneath the varnish.
You should apply tung oil first in the hope it will highlight the grain better. You might try a thin coat of varnish on bare, scrap wood first and skip the tung oil layer altogether after comparing the effects.
Assuming you are talking about 100 per cent pure tung oil, try mixing three parts to one part oil-based polyurethane and add a little Japan drier to help with drying. This mixture will give your tung oil a little more build and protection, while maintaining an easy-to-apply, wipe-on finish. Plus, using the same mixture for all coats will prevent the finish from cracking due to unequal flexibility of different finishes when the wood expands and contracts.