How often does exterior polyurethane need reapplication?

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Forum topic by JSThoreau posted 08-04-2016 02:21 PM 924 views 0 times favorited 2 replies Add to Favorites Watch
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1 post in 863 days

08-04-2016 02:21 PM

Topic tags/keywords: finish spar urethane t g raw pine

I just finished sanding down and finishing a T & G exterior ceiling with helmsman spar urethane, and the homeowner was curious about how often she needs to plan on reapplying, and also how to maintain it.

The ceiling is over an open outdoor patio area, and is a finished and shingled roof above, so it does not get any direct weather contact. I don’t imagine it would need to be reapplied regularly, not anytime sooner than every several years. I have heard even poly floors with regular foot traffic can go for five years or more without needing reapplication. Am I on the right track advising her along these lines?

She wanted it finished because it appeared to be raw knotty pine t & g (when I got up there to clean and sand it, it appeared there was a very thin layer of something, whether that was linseed oil or some such), and had been weathered, and discolored with dark moldy-like patches throughout, along with a lot of cracking, and shrinking. She wanted something on there that would help prevent that weathering and cracking from getting worse.

Also, is any maintenance necessary? I imagine the air is responsible for depositing the mold and weathering (the worst of the weathering was on roughly the north side), and so is it worth having someone come and wipe down with a damp rag once a year or some such gesture?

Thanks for any help!

Jacob, Portland Oregon

2 replies so far

View mahdee's profile


4021 posts in 1969 days

#1 posted 08-04-2016 03:20 PM

Hi Jacob,
I think it should hold up well as long as there are no sun exposure on it. I use the spar Urethane on the door on my shop which has direct exposure to the elements and it didn’t last for more than a year. I think your assumptions are accurate.


View JBrow's profile


1366 posts in 1122 days

#2 posted 08-04-2016 05:51 PM


A ceiling is a little different from the experience I can share regarding renewing Helmsman Spar varnish, but perhaps it can be helpful. We have a west facing covered patio. Setting on the patio is a coffee table and an end table both with a red oak tops (base is metal). When I finished the table tops I applied 5 coats of Helmsman Spar varnish on all six sides. This year, two years after being completed, I applied a renewal coat of the spar varnish. I did so only because the gloss sheen had become dull.

The table tops get far more weather than the ceiling of the patio cover, so I would expect the spar varnish to hold up a little better on the ceiling. Therefore a renewal of the ceiling I would think would be less often. I would be concerned that if not renewed often enough, the UV bouncing up to the ceiling could lead to cracking and peeling. A periodic renewal coat when the finish becomes dull therefore is probably a good idea. I would guess renewal would not be required more often than every 3 – 5 years, but annual inspection is probably better.

I would guess that the cracking and shrinking is due to only one side of the ceiling being finished. Had all six sides been coated with a sealer, the cracking and shrinking would probably have been less.

The dark moldy patches are a mystery to me. I maintain a red oak picket fence annually and notice what I think is mold in the wood. Bleach and soap does little to remove it. I have yet to figure out why it occurs on the sealed fence, how to prevent it, and how to remove it without deep sanding. My theory is that the sealers I have used are penetrating oils, thus leaving places where mold can get a foothold and then grow. If this is correct, then a film finish like a spar urethane should prevent its re-occurrence – but I really do not know.

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