putting spar varnish on top of urethane varnish?

[dustin510]

If I stained, then put 3 coats of a urethane varnish on, can I come and put a spar varnish on top of that for extra protection? the urethane varnish isn't rated for outdoors so I want to really seal it in with the spar varnish.

The urethane varnish I put on is the bottom can, the spar varnish in question is on top. I'll probably go and return the gloss for satin.

 

[smallboat]

Dustin, did you mean to include a photo?

Aside from the usual oil/water based compatibility issues I don't see any reason you can't do what you're thinking.
If the Spar varnish is oil based like the one I use it should be fine.
Just make sure you scuff up the urethane so there's some tooth for adhesion.

I generally don't use oil and water based products in the same finish, but my understanding is oil can go over water but not the other way 'round.
I think of oil floating on water and that's how I remember- sure hope I got it right!

 

[KenOfCary]

I agree with smallboat. As long as they're both oil based products you should be fine. In fact I thought SPAR Varnish was just a robust flavor of Poly, but I could definitely be wrong on that.

 

[smallboat]

well Ken you set me to wondering and we all know where that leads-
Our good friends a wikipedia have this to say

"Unlike drying oils and alkyds which cure, after evaporation of the solvent, upon reaction with oxygen from the air, true polyurethane coatings cure, after evaporation of the solvent, by a variety of reactions of chemicals within the original mix, or by reaction with moisture from the air."

I like to use Z-Spar captains varnish, its a combination of alkyd and phenolic resins in a linseed/tung oil base. So I guess its not a polyurethane but is a Spar varnish.

Our wiki friends categorize spar varnishes as having two key properties- UV protection and flexibility.

sorry to get off track Dustin!

 

[KenOfCary]

Great information - I guess that SPAR varnish is more Alkyd or resin varnish than it is a Poly. I learn something here every day.

To the original question though I'm guessing (and still just guessing) that Oil and Oil will mix just fine. The flexibility part worries me a little. If the poly is not as flexible then will an overcoat of SPAR make it so. This is the feature that allows for expansion/contraction of outside items during extreme humidity changes.

I really don't know at this point so I defer to the experts here.

 

[CrealBilly]

No expert by any means... reading and learning. It's interesting to me what our wiki friends say and speaks to why my favorite finish is 50/50 acetone/oil base poly. Goes on easy / dries quick and can be colored to any shade by mixing in oil based artist pigments.

 

[smallboat]

I wouldn't be surprise if somebody is marketing a urethane with UV enhancements as a "spar" varnish.

As for the spar varnish "reaching down" into the urethane and influencing its flexibility, I couldn't say.
That's a level of organic chemistry a course or two beyond me- but I know where we can ask one!

The flex in spar varnish was to acount for actual flex of the wood. Think of a sailboat mast or boom straining under the load.
They bend, they're designed to.
the finish has to be able to stay adhered.
I guess the same property would cover for a reasonable amount of expansion and contraction.

 

[Jeff]

Years ago I naively put water borne polyurethane on an outdoor table. What a disaster!

It cracked, peeled, and generally came apart. Lesson learned after some serious reading: Polyurethane is hard, brittle, has poor uv resistance, and blows apart with seasonal extreme changes in temperature and humidity.

Enter a high quality marine spar varnish (Epifanes) which is flexible, won't crack and peel, and it has excellent uv resistance. :thumbs_up

http://www.jamestowndistributors.com/userportal/show_product.do?pid=92

I'm not clear on what the OP has used. I'm guessing that urethane varnish is confusingly different from polyurethane. This article does help to clear up some of the confusion.

http://www.thewoodwhisperer.com/articles/difference-between-spar-varnish-and-regular-varnish/

 

[bluedawg76]

If I stained, then put 3 coats of a urethane varnish on, can I come and put a spar varnish on top of that for extra protection? the urethane varnish isn't rated for outdoors so I want to really seal it in with the spar varnish.

The urethane varnish I put on is the bottom can, the spar varnish in question is on top. I'll probably go and return the gloss for satin.

it's not really a question of "sealing the wood". The wood will breathe (accept/lose moisture) to equilibrate w/ the relative humidity. A finish just slows the process, but does not stop it. Spar varnish won't behave any differently than polyurethane (oil cooked w/ urethane resin) in this respect. The difference as Jeff pointed out is that spar varnish is softer and more flexible than polyurethane in part due to the oils used. Spar varnish will (supposedly) accommodate the larger expansion/contraction of the wood as it breathes w/ the larger humidity swings that occur outside, whereas other finishes like polyurethane may crack. Then of course there are the UV inhibitors added to the spar varnish that are not present in indoor finishes which may also significantly contribute to the differences. In theory, you may run into trouble w/ poly on outdoor furniture b/c the poly may crack -irregardless of spar varnish on top, but that said, I've not tried it, so unless you want to scrape off the current finish, I'd say go for it.

 

[Roy G]

Have you checked with the manufacturer? They should be able to tell you whether it will work or not.

Roy G