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What is the proper way to dilute water based oil-modified polyurethane?

I know for oil based poly, to use mineral spirits. Water based poly, use water. I couldn't find any details on how to dilute water based oil-modified polyurethane though. Water or mineral spirits?

I'm struggling keeping my finish bubble free using a foam or bristle brush, and want to dilute it thin enough to wipe on.

  • Hi and welcome. You couldn't find details I think because you basically don't dilute waterbased varnishes. They're totally unlike oil-based products which can literally be diluted to any amount. In short, you can't really convert waterbased varnish into waterbased wipe-on. You can try the minimum dilution ratio used for making wiping varnish, which is about 1 part in 4, but it won't work the same way and won't allow the product to be used in a similar manner. The manufacturers specifically warn against dilution beyond quite modest levels, like maybe 10% at max, depending on the exact product – Graphus May 31 at 8:32
  • That being said, people do do this, sometimes even adding surfactents intended for latex paint. No idea how "oil modified" finishes will behave if you do this, but I suspect you are on your own; you could try an experiment on some test pieces. But, as Graphus points out, this is just not recommended with water-based finishes. – jdv Jun 1 at 1:05
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We may as well turn the comments into an answer.

What is the proper way to dilute water based oil-modified polyurethane?

Unfortunately, the official answer from the manufacturers is "you don't". Though some folks have used distilled water and even surfactents intended for breaking surface tension in latex paint (usually for spray application) it is unclear how oil-modified finishes would work with either. My guess is probably not well, but you are encouraged to try on a test piece if you want.

Otherwise, your solutions are:

  1. Get a wiping poly and use that if this is your preferred application method.
  2. Review the application notes from the manufacturer and try again.

As for (2) they probably recommend:

  • Apply with a synthetic brush for water-based finishes and not a foam brush.
  • Wet the brush with a solvent (water, in this case) and shake off excess before using it to apply finish.
  • Never shake the finish; stir gently only.
  • Don't wipe excess off the brush along the container. Rather, lightly tap the brush a little against the side of the container to remove excess so the brush stays relatively loaded. This may be easier if you transfer some of the finish to another container and use that as your working amount.
  • Brush with the grain.
  • Apply the finish and move on. Try not to go over these fast-drying polys.

The idea is that we want the finish to go on relatively wet so any small bubbles rise to the top before it dries. Once dry enough, a light sanding should be enough to rough up the surface and remove any small bubbles before applying another coat.

If you read between the lines of the application notes, you can see that since they recommend wetting and cleaning the brush with water, you can probably dilute this finish some small amount with distilled water. Whether or not you should is up to you.

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  • Another one where you totally kill it IMO *thumbsup* – Graphus Jun 4 at 14:52
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One trick I've used with success, because yes, water based polys can be foamy, is this: Once you lay down your coat, and tip out as many bubbles as you can, take a hand held propane torch and wave the flame over the surface. Obviously, you don't want to cook your finish, but a pass of hot air will pop bubbles in a satisfying way.

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