Thinning Spar Varnish

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Forum topic by slick1 posted 05-09-2010 02:27 PM 13796 views 0 times favorited 8 replies Add to Favorites Watch
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19 posts in 3443 days

05-09-2010 02:27 PM

What would be the best type of thinner & ratio for 1 US gal. of OLD MASTERS MARINE SPAR VARNISH, high gloss. Brushing onto 5/16” knotty pine V-joint on interior of outdoor shed.

-- - - - Rod , Lloydminster, Saskatchewan, Canada

8 replies so far

View CharlieM1958's profile


16283 posts in 4695 days

#1 posted 05-09-2010 03:24 PM

Mineral spirits should work, although I wouldn’t try to thin it too much….. maybe 1 part thinner to 10 parts varnish. The more you thin it, the more you are reducing the protective properties of the varnish.

-- Charlie M. "Woodworking - patience = firewood"

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1381 posts in 4605 days

#2 posted 05-09-2010 04:29 PM

e pluribus unum.

-- 温故知新

View John Gray's profile

John Gray

2370 posts in 4362 days

#3 posted 05-09-2010 04:37 PM

I agree with hobomonk above. I just thinned some “and the can said” thin 20-50%, I thinned about 30% with paint thinner and it worked fine. The thinning really helped with a shorter drying time.

-- Only the Shadow knows....................

View JJohnston's profile


1622 posts in 3768 days

#4 posted 05-09-2010 04:39 PM

hobomonk said: “I would suggest following the instructions on the label and/or referring to their website.”

Interesting that you didn’t say that when Zinsser said not to use polyurethane on top of their sealer. Must be that “internet expertise” we’ve all heard about.

-- "A man may conduct himself well in both adversity and good fortune, but if you want to test his character, give him power." - Abraham Lincoln

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19 posts in 3443 days

#5 posted 05-10-2010 12:50 AM

Thanks all !
Great read Shopguryl. Learned alot.

I read somewhere that Japan Drier is ok to use with spar varnish , but is it ok to put Japan drier in with a thinned spar varnish ??

-- - - - Rod , Lloydminster, Saskatchewan, Canada

View Scott Bryan's profile

Scott Bryan

27250 posts in 4299 days

#6 posted 05-10-2010 12:51 AM

Rod, it will brush on without thinning but this is somewhat prone to runs and sags.

If you want to thin it out in order to make it flow easier then Charlie’s comment is pretty good. You can thin it from 10 to 50 percent with mineral spirits but, the more you thin it, the more coats you will need to apply to get an equivalent build. It should work just fine for your application. If I were doing something like this I would apply at least two full coats. I have pine exterior doors that I originally finished with spar urethane over 20 years ago. All three doors are exposed to the sun and I have refinished them only once in the ensuing period.

-- Challenges are what make life interesting; overcoming them is what makes life meaningful- Joshua Marine

View hObOmOnk's profile


1381 posts in 4605 days

#7 posted 05-10-2010 03:53 AM

Japan drier doesn’t need to be added to thinned varnish if the undiluted varnish drys okay without it. Thinning the varnish doesn’t change the ratio of active components, it only dilutes them. Curing (polymerization) of the finish occurs after the solvent evaporates.

Hope this makes sense.

-- 温故知新

View WestcoastTony's profile


5 posts in 2889 days

#8 posted 10-19-2011 01:15 AM

I’ve mentioned this on another thread already, but it might be worth bringing it up here. In California there is now a substitute for mineral spirits that has a mix of acetone and petroleum spirits. I recently tried to thin spar varnish with it and got a strange buttery matte finish. A chemist friend suggests the acetone is causing the resin to separate out of the varnish. This kind of mix should be avoided.

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