Oil based spar urethane over water based spar urethane

[davidbogg]

Hello all. I am very new to wood finishing. Recently I had fresh cypress garage doors installed and we finished them with two coats of water based spar urethane. We were hoping to get that warm glow that we have on our front doors; however, I just learned that water based spar urethane only gives a clear coat and not the amber affect that an oil based spar urethane gives.

I am thinking of putting a few coats of oil based spar urethane over the water based spar urethane. The water based spar urethane has cured for about 2 weeks so far. Will this work to be able to get the desired look? Or will the layer of water based spar urethane keep the oil based product from interacting with the wood?

Would it be best to try and strip the water based spar urethane off first and then recoat the cypress doors with the oil based spar urethane to be able to have the oil nature of the product best react with the wood? (does the oil actually interact with the wood?)

 

[Bill Clemmons]

I'll respond to this, but hopefully Howard Acheson (our resident finishing expert) will chime in as well. I trust his opinion more than my own.

Assuming the water based urethane is completely dried, you should be okay to go over it w/ an oil based urethane. Once the carrier (water) evaporates, the urethane is pretty much the same as any urethane. The reverse, however, is not true: you can't put a water based urethane over an oil based urethane.

A couple of things I would consider first: given the high humidity in Florida, I might wait longer than 2 weeks for the spar urethane to completely dry. Second, you might want to lightly scuff the surface w/ sandpaper to improve the adhesion of the next coat.

Bill

 

[davidbogg]

Thank you Bill. That helps a lot. I was concerned that the oil of the spar urethane might not be able to penetrate through the layer of water based urethane. I didn't know if there actually is an interaction of the oil with the wood too or if it just coats the wood. Good to learn that putting oil based over water based is ok.

 

[Bas]

Some speculation here, based on some experience with both oil and water based spar vanishes:

The water based urethane should be dried and cured by now. You will need to scuff sand the doors with 220 grit for proper adhesion. This would be the case even if you put water based over water based (and usually oil based over oil based too). The oil based varnish will give you the amber look you want, although it may not be as "deep" because of the water based coat in between. But I doubt you'll see much of a difference. You may want to test in an inconspicuous place first and see if you like the end result before you spend a lot of time (and varnish) doing the entire door.

The good news is that in two years, you get to reapply the finish, and get it exactly the color you want

 

[Howard Acheson]

Yes, you can over coat a fully dried waterborne varnish with an oil based varnish as long as you prepare the surface correctly. But all finishes have lessened adhesion to any urethane finish. Therefore, you need to scuff sand the existing finish thoroughly using 280 or 320 paper. Sand in the direction of the grain.

Now, the real point. Polyurethane rapidly degrades with long term exposure to the UV in sunlight. It will quickly become cloudy, then begin to crack and peal. In spite of what it may say in the literature or on the label, polyurethane varnish has poor performance outside. To have the best chance for a longer lasting finish, use a non-poly exterior marine finish. You will not find these finishes at your local big box. Go to a marine supply store or a marina and look for brands like Pettit, Wolsey, Interlux or, best of all, Epifanes. Follow the directions on the label.

An option is to use a "clear paint". Go here for info. I have used this on a number of garage doors and it holds up very well.

http://www.hardwoodlumberandmore.co.../75/ArticleId/26/Paint-On-A-Clear-Finish.aspx

 

[davidbogg]

Good stuff. Thank you. We coated a redwood door with the water based spar urethane and it turned the door a slight amber color. We were not expecting this. We wanted it to keep its lighter color. Will the product you mentioned help keep it lighter?

Also, the reason we want it lighter as we want it to match the finish of cypress garage doors that we have.

Any insight is greatly appreciated.

 

[Howard Acheson]

Normally, waterborne clear acrylic finishes are water clear. In the last few years, some formulators have be adding an amber dye to their product to mimic the warm appearance of an oil based finish. This is particularly prevalent with waterborne floor finishes.

The marine products I mentioned above are oil based andcontain UV inhibitors. Oil based finishes are amber in color and the UV additives will add or increase the amber color of a finish.

If you use an acrylic "polyurethane" waterborne exterior finish, you will experience lessened durability as noted above.

Let me also say that you should always initially apply a finish to an inconspicuous spot to see how it will look.

 

[Jeff]

I'll be the devil's advocate once again. Shouldn't we be advising David entirely against the use of urethanes for his exterior cypress or cedar doors for all of the reasons mentioned above in the responses? He's new to this and has already made the urethanes plunge so how should he salvage that misstep? Lots of alternatives exist that can achieve the desired look and provide superior exterior durability and UV protection. :icon_cheers

+1 to recommending Epifanes Marine Spar Varnish (diluted about 4:1 with MS), a variety of Cetol products in clear and tinted, as well as Waterlox Marine finishes.

http://www.paintsource.net/pages/products/sikkens/door_and_window/cetol-door-window-brochure.pdf

http://www.waterlox.com/products/

I'm a PITA once again, but I'd be remiss if I didn't state the obvious!