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I was going to ask this question after getting some misleading information from my can of poly, other questions and answers here and from my own experience.

What I never understood was why should I not shake the can? The instructions from Minwax's website don't really explain why, but state:

Stir Minwax® Fast-Drying Polyurethane before and during use to eliminate settling on the bottom of the can. Stir in such a manner as to rotate the product from the bottom to the top of the can. NEVER SHAKE

emphasis mine.

Barring spilling the product all over the place; what am I risking if I shake the can?

FreeMan
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Matt
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4 Answers4

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Apparently it's the bubbles

The concern is that the agitation of the can will create bubbles. Brushing poly will transfer those bubbles to the work and there will be a good chance they will dry/cure in place. That would of course ruin the work or extend the finishing process as you would have to sand an reapply.

That is sort of right.

There is a great article that Graphus linked my too that covers 7 Myths of Polyurethane. No sense copying the entire portion of that post here but the main points are:

  • It is true that bubbles can cause issues. Bubble creation is inevitable though whether you shake the can or not.
  • Using the brush you are going to create bubbles regardless with agitation.
  • Most bubbles will naturally pop. A recommended step with poly is to lightly brush back (also called tipping-off). That should remove the rest of the bubbles.
  • If you still have bubbles after that you can still lightly wipe something like mineral spirits over the surface to dissolve the bubbles. Used in moderation this will not negatively affect any of the finish work done thus far. Worst case a light sanding with very fine sandpaper (400 or greater) should help. In my experience I would avoid sanding as to not remove too much finish by accident. Rarely would I still expect bubbles at this point).

You are going to get bubbles in any event. Does that mean you should run the can of poly in a paint mixer? Probably no. However as long as you know how to deal with the bubbles and clean them before the finish sets you are not going to have any issues.

Community
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Matt
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  • Hopefully I have not omitted anything important. That is directed at you Graphus :). Rotating that can in my hands because I could not shake it bugged me. – Matt Nov 29 '15 at 05:18
  • This will be a good answer when we hear from someone who has shaken polyurethane and successfully brushed out the bubbles. Seems to me that it might be a bit of trick to eliminate what is likely foam as opposed to a few bubbles. – Ast Pace Nov 29 '15 at 05:35
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    On the Garco cans it says do not shake, and specifically states it is to avoid bubbles. I think it is probably even more of a problem when you're applying it to huge surfaces with a big pad or something, like a hardwood floor in a gym, and it isn't convenient or feasible to go over it to brush out the bubbles. But, I have actually shaken them as an experiment and with a brush it leaves more bubbles but with foam or a rag it is no change. – Jason C Nov 29 '15 at 05:48
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    (And no it isn't foam, it legit puts a bunch of bubbles uniformly spread through the can, but they'll come out if you leave it set a bit.) (And yes even the additional bubbles with a brush are super easy to get out, nothing beyond the normal light wipe.) – Jason C Nov 29 '15 at 05:54
  • @JasonC It looks like you have provided the experience that I was asking for. +1 to Matt – Ast Pace Nov 29 '15 at 06:04
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    Important to make the distinction: waterbased finishes and what I like to think of as real poly (oil-based) are completely different in this regard. – Graphus Nov 29 '15 at 10:51
  • @Graphus This only applies to oil based poly then? (I've only used oil based poly) – Matt Nov 29 '15 at 13:59
  • @Matt Fwiw Minwax polycrylic has the same instructions and same effect when I shook it. Lots of thick things have that advisory; resins, epoxies, etc. – Jason C Nov 29 '15 at 14:50
  • @JasonC I just knew something was up when I brushed and I was making bubbles. – Matt Nov 29 '15 at 16:11
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    Bubbles are a major problem only with waterbased poly — waterbased finishes in general, because of their tendency towards bubbles (called foaming in the industry, as you can actually create foam at the most severe). Broadly, oil-based finishes have a long enough drying time that any bubbles have sufficient time to release on their own (plus there are fewer anyway and they are less easily created). Additionally with the fairly common wipe-on approach these days bubbles are practically a non-issue, which is one of the great things about wiping oil-based that makes it nearly foolproof. – Graphus Nov 30 '15 at 08:25
  • If the can has bubbles in it, you can leave it open in a vacuum chamber to get rid of most of them. You all have vacuum chambers, right? – Samuel Dec 01 '15 at 18:51
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I shook a tin of wood varnish and the lid blew off and hit me in the face. I had a few scary minutes of vigorously washing varnish out of my eyes and off my skin. If it says don't shake, best do as you're told.

Han.banan
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I have been told that it isn't so much that you get bubbles as Matt said but that the bubbles become much smaller and more of them when shaken, making them more difficult to brush out and before they dry, or set. It is more of a problem with gloss poly because it can make the finish look cloudy. I think it is the same with varnish and shellac.

Duckweather
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I work for The Home Depot in the paint department, and the cans say STIR, DO NOT SHAKE. Bubbles are the issue, if you don't use a brush. From my personal experience, i have had bubbles either way. It has been true in my experience that if you lightly brush back the bubbles can go away. I have also lightly run across the surface with Mineral Spirits to dissolve the bubbles, and that works. Overall I wouldn't choose to shake an oil based poly, because it creates more bubbles than you'd want, but there are ways around it. To get the best results, just follow the directions on the can. I can't tell you how many times i've explained what to do to get the best results from a product, and customers don't listen, and they come back and say "this product didn't work". Just follow the "RECOMMENDED" directions from the product; it's there for a reason, but to each his own.

Briana
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