Rot Doctor

[Miscellaneous Q&A Index]

Subject: Spar Vanish ? (cedar on underside of patio roof)

Date: Monday, April 13, 2020


This cedar (see pic) is going to go on the underside of a patio roof. I need a good, oil-based clear coat on this. What product do you sell for this? I see that System Three offers a Spar Varnish so will that give me the long-life protection that I need? Do you sell this product?


Cedar ceiling boards


We do sell the System Three marine varnish. Applied properly, it should provide the protection that you are looking for.

The two enemies of varnish are sunlight and moisture intrusion. Since the cedar will be placed on the underside of a roof, it is not likely to get significant UV damage. My main concern would be moisture. I would advise coating both sides equally, to prevent uneven movement of humidity from causing cupping of the wood. The other concern would be if you were to paint or varnish the back side, and varnish the visible side, then install with nails or screws, the penetration of the fastener could become a place where moisture could get past the varnish and stain the wood, and possibly cause lifting of the finish over time. Since the cedar would be under the roof, then rain is not likely to be a cause of moisture intrusion, except for possibly at the edge of the roof. Dew would be my main concern, if that is a regular occurrence in your area.


[Miscellaneous Q&A Index]

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