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How do I keep air bubbles out of my varnish?


Konrad

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It's like a zen thing, or something.

I just *can't quite* figure out what the problem is.

I don't use the most expensive brushes, but I'm definitely not using that cheap Mexican and Chinese crap either.

What causes the small air bubbles...?

The stroke speed of the brush seems to not matter.

The angle of attack of the brush matters for other things, but not for avoiding air bubbles.

I'm using gloss spar varnish by Minwax, I assume that's an OK product.

I don't get it.

There's some voodoo secret, and I want to know it.  >:(

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Were you varnishing bare wood and was it in a place where the sun might get to it?  Or were you maybe using a foam brush?

I've had trouble with epoxy (but not necessarily varnish) bubbling when applied in the morning while the ambient temperature is rising and I've been a victim of cheap foam brushes (this time with varnish).

Varnish is evil.  It doesn't like me at all.

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You wouldn't be stirring that varnish would you? With gloss varnish there's no need and in fact the stirring can introduce bubbles in the varnish.

There's nothing in gloss varnish to NEED stirring - carefully pour enough for the job into a clean container, dip the brush in, DON'T wipe it hard, and flow on the finish. Don't pour the leftovers back into the can.

Of course Laura calls me a pig with a brush, but I know what I see HER do. I varnish with a spray gun :D

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With today's varnishes, I don't ever have a problem with air bubbles unless the finish is put on in an increasing temperature situation like Ken mentioned.  It's a no-no to either varnish or epoxy coat where the rising temperature can cause outgassing from the wood.  Varnishing in the morning on a sunny day is a sure bet for bubbles. Normally the bubbles left by a brush will quickly disappear.

Best to varnish in a stable or falling temperature.

Some painters used to chill the varnish, never stir or shake it, dip and remove the brush very carefully and flow it on to avoid adding bubbles.  I prefer to have the varnish at least as warm as ambient and let the bubbles break naturally.  It works for me and I get tons of initial bubbles from my sloppy brushwork but they all go away.

If your varnish calls for a reducer to be added, that will help the varnish to flow out and break bubbles also.

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If I am not mistaken, Minwax Spar is a polyurethane.  McKloskey is now a subsidiary of Minwax and it is actually a varnish.  I have had mixed results with polys and have never been able to isolate the causes.  I always have better luck with varnish if I cut it a bit with thinner, usually mineral spirits.

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The air bubbles I'm dealing with aren't a product of off-gassing, but a result of application.  They're right there at the trailing edge of the brush during the application, and they don't break.  I may try the thinning thing, although I've always been reluctant to thin a varnish.  Thanks for the input.

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MinWax Spar is a polyurethane and says so right on the can just under the big yellow letters that says "Helmsman".

Assuming you're not out gassing, then it's your technique and/or the viscosity of the material. Use about 10% Penatrol which is a "wetting" agent used in oil paints and available at the hardware store or a painters supply. This will improve the "flow" rate of the paint so it will level better and the bubbles will pop easier too. If you use more Penatrol, use no more then 20% as it will affect the gloss (it'll go satinish) though the durability will not be harmed.

MinWax spar has a bunch of additives and is a product intended for the "home owner" type of use. It has pretty good UV inhibitors, but it's not especially durable or hard, compared to other polyurethanes and no match against the high end stuff like Bristol or Epifanes. It's main advantages are cost, which is moderate and availability in home improvement and hardware stores.

As Tom and others have mentioned, you have to "flow" the product onto the surface, not brush it. A common issue is over brushing the stuff, which produces bubbles. Flowing is difficult to describe well. The material just runs down the brush, onto the work and is sort of pushed into position and left alone to dry. This is hard to do in a uniform layer, but with practice it's easier. Good brushes make a huge difference over inexpensive ones, because they flow better and usually hold more material (with control).

Don't worry about your first four coats anyway. These are just "bulking" coats which will be mostly sanded off. Just get some material down as evenly as you can then level the surface with block sanding when it's good and cured. Only the last few coats have to be applied with flawless applications, as these are the finish coats and what everyone will see. The bulk coats (typically more then twice as many as the finish coats) provide a base to protect the wood and offer a "safety zone" that you can repair against when future refinishing becomes necessary, as the finish coats get damaged or wear out.

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I'm amazed he can get a season from it. We tried several cheaper "spar" varnishes and found ourselves ( or Laura anyway) refinishing after just a few months. Even Z-Spar 2015 won't last a full season here in our sun. We've had some others fail in a matter of weeks.

Now the Bristol Finish Laura put on the mast and boom last September is still looking great. Of course the boom is covered so that's no real surprise, but the mast is exposed full time.

We recently tried some Rust-Oleum Spar varnish- THAT failed in two weeks.

Now Min-Wax Polyurethane is good stuff- we'd had it on the floors and walls of the house for 12 or 13 years now and it still shines ( except where my computer chair rollers ride) but it ISN'T for exterior use.

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Thanks for the replies.  I learned something here, and that was the point of the post: To learn from others.  Minwax's varnish is indeed a poly, and I never claimed it wasn't.  Hell, I don't even know what truly decent brushes are.   ;)  

I just pick it up at the hardware store, and (for the most part) looks just fine on my boat.  My boat lives mostly indoors, so my humble Minwax lived through 4 years of summer use and 4 more of dry storage quite admirably (except for the taffrail).  Maybe it's because I like to varnish, and I put a lot of coats on..?

I may try the thinner.  

Or I may just quit being such a hand-wringer about it.

I'll let you know what becomes of it.

I have a new cockpit sole that's about 75% complete now, and it's those big flat surfaces that are making me annoyed.  

The smaller brightwork is all good.

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Ummm, Konrad, you do know where those varmits (short people, rugrats, etc.) come from, right? ;D

My daughter just figured it out after the 4th one but her older brother only took a couple.  Time will tell what my 3 younger "resource suckers" will do.  Oh, yeah, it took my wife 5 of them when she said that's it.  I am not sure I know the answer myself. ;D :P

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Ummm, Konrad, you do know where those varmits (short people, rugrats, etc.) come from, right? ;D

My daughter just figured it out after the 4th one but her older brother only took a couple.  Time will tell what my 3 younger "resource suckers" will do.  Oh, yeah, it took my wife 5 of them when she said that's it.  I am not sure I know the answer myself. ;D :P

Other people should have lots of babies ... I'm still thinking there might be Social Security by the time I retire, but that only works if there are people paying into it.  I tell all the young couples I meet to BREED!

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I've been away for a few days to the Georgetown woodenboat show and came back here to learn more about varnishing. 

Varmints, rugrats, short people ???  Do they help with the varnishing ???  Do you dip the varmint and drag it over the work ???  Do they bite or do you drag them by the tail ???  If you do it that way, do they leave tooth marks ???  How do you clean them ???

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I've been away for a few days to the Georgetown woodenboat show and came back here to learn more about varnishing. 

Varmints, rugrats, short people ???  Do they help with the varnishing ???  Do you dip the varmint and drag it over the work ???  Do they bite or do you drag them by the tail ???  If you do it that way, do they leave tooth marks ???  How do you clean them ???

Gee, Tom, I didn't think you had much left to learn on that subject.  I usually look to you for learnings. :)

Are you just fussing because this thread strayed a bit?  It will stray back when someone has more varnishing things to tell about.  Do you have any to share?

Meanwhile, how was the boat show? ;D

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