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Is polyurethane only for stained wood, and not painted wood?

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Q. I am attempting first woodworking project and need some advice. I am building a railing with a stained hand rail and base, but the spindles are painted white. I applied polyurethane on the stained wood to give it a high gloss and provide protection. Can I polyurethane over the white paint as well to make the paint job more durable? Is polyurethane only for covering stained wood?

Cindy Leaf
- Springfield, Virginia


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A. I've used polyurethane on both bare wood and on painted metal with good results. No inherent reason it wouldn't work on painted wood. First try the polyurethane in an inconspicuous spot to make sure it doesn't dissolve the existing paint. If it does dissolve the paint, you *may* be able to use several very light coats initially to seal the paint surface. In either case, prep the existing paint as you would for any other re-paint. For instance if the existing paint is glossy be sure to sand or etch it first, and as always, make sure it is clean. Do keep in mind though, that polyurethane is somewhat difficult to strip for that 'next' paint job.

Tom Gallant
- Long Beach, California


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A. I've never done anything like that myself, but I can offer an educated guess. Most likely it will NOT work, polyurethanes are mostly stains versus surface finishing. The polyurethane works by working it's way into the wood and then hardening, causing a good weather repellant coating.

It could work, but most likely not, it will just sit on top of the paint and won't really offer any added protection. I suggest stripping down the paint on there and use the polyurethane on the whole thing.

Marc Banks
- Elizabeth City, North Carolina


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A. I personally would not coat white with most polyurethanes. The majority of the urethanes you come across are not UV stabilized...when exposed to UV rays(sunlight)they will yellow with age. This isn't very apparent when coating warm colored woods but it will make your white look pretty nasty. They do make special urethanes that you can mail order that are UV stable..but they are expensive..if my memory is correct I think I had to pay about $90 a gallon.

Jason Aube
- Flint, Michigan


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A. You can buy UV inhibited polyurethane, but it is not clear! It is for exterior work and is a sick honey color right out of the can.

James Watts
- Navarre, Florida

October 18, 2010

Q. Is it appropriate to polyurethane a stained wood porch railing that gets direct afternoon sunlight?

Shelley Fialho
- Dallas, Texas, USA

October 19, 2010

A. Hi, Shelley. Based on the above, I'd say yes. We polyurethaned an outdoor bench for the very rough service of our sun-drenched deck on a salt water lagoon, and it lasted at least several years. I also see wood patio furniture for sale, advertised as finished with UV-resistant polyurethane. Sounds fine, good luck.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

Outdoor Polyurethane


February 4, 2013

Q. I would like to know if a person could use polyurethane to on a deck. I have tried all types of stains and they just don't last and I had just thought about doing this. Any suggestions?

Aleta Fanning
- Green Forest, Arkansas, USA

February 10, 2013

A. Hi Aleta.

I'd be concerned about it getting too slippery when wet. Not to recommend a brand, just a concept, I feel you'll be okay with stain every 3-4 years, and something like Thompson Water Seal every 2 years (better yet, every year if you're ambitious; it's transparent and if you run out of time partway through, it won't matter if you're trying for every year.)

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
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