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I've seen a lot of tips about cutting the first coat of polyurethane with mineral spirits to reduce drying time. However, given that it's the first coat, I have a full can of polyurethane. I'd pour some polyurethane out into a separate container, but am worried this will create air bubbles. Likewise, I imagine the act of adding mineral spirits to polyurethane could also introduce air bubbles.

How can I add mineral spirits to polyurethane without creating air bubbles?

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I add 20% thinner to my polyurethane to promote even coats, prevent bubbles(they pop more easily on their own when thin), and speed up drying time. You can alternatively use lacquer thinner and this will dry even more quickly due to it's lower flash point -- but in high temperatures it might dry too quickly.

As far as methodology for mixing, I have a "mixing can" that I bought at a big box store -- they sell empty paint cans -- and I add 20% thinner and the rest poly. I use a plastic can attachment that acts as a spout to prevent poly from filling the lid gap when pouring. Stir with a stir stick. I don't get too many bubbles but I don't worry about them either because they pop on their own. Pour onto the side of the can to prevent bubbles (think of pouring beer from a bottle into a glass).

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Edited my title based on your answer; I will try your lacquer thinner suggestion tonight as my can of mineral spirits is almost empty and I'm in a bit of a time crunch and anything extra to speed up the dry time will be helpful. – virtualxtc Mar 12 '14 at 17:21
Great, let me know how it goes! – Ethereal Mar 13 '14 at 14:43
The lacquer thinner causes the poly to turn milky and settled at the top. I stirred for a bit then decided to add more polyurethane. It seemed to clear up then. I started the job, and about 45 min in there seemed to be some phase separation going as the shallow end of the paint pain fairly clear. The coat was still tacky 2 hrs in, so I waited until hr 4 before adding coat 2. – virtualxtc Mar 14 '14 at 3:16
Also, I found the lacquer thinner cleaned out the paint tray much more effectively than mineral spirits did. – virtualxtc Mar 14 '14 at 3:18
I typically stir every 20 minutes, especially if you aren't using a gloss finish -- the flattening agents will settle to the bottom as well. – Ethereal Mar 14 '14 at 13:40

I normally would not recommend thinning urethane. If you have a new can of fresh urethane, it should be ready to go. I would try to apply it thin rather than a thicker coat. If you feel you really need to thin it, just slowly pour some in another container and add a small amount of thinner at a time. Stir it slowly and you will have no issues with air bubbles. Never shake or use a high speed mechanical mixer. If you see some small bubbles, don't worry. They should dissipate during application. the only time air bubbles are an issue is when the product is shaken hard and the urethane appears real cloudy.

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+1 Thanks for the info about small bubbles not really being a problem. Perhaps it's the type of urethane I have, but if I try to spread to thin I tend to get streaking. – virtualxtc Mar 12 '14 at 17:11
The streaking is because of the brush, not the product. use a foam brush if the surfaces are fairly flat and a good quality china bristle brush on most anything. Don't use a nylon brush. Another hint is to not overwork the urethane. urethane is fairly self leveling, so it you brush it too much, it has a tendency to streak. Be sure to sand lightly with 400 or finer between coats, clean off all dust before the next coat. – shirlock homes Mar 12 '14 at 19:46
Why do you say sand with 400? the can says 220? Also I hear that you can skip sanding between coat 1 and 2 if they are laid down less than 10 hrs apart (which is what I did last night). – virtualxtc Mar 14 '14 at 3:10
skipping the sanding after coat one is the worst advice I can imagine. The first coat often raises the grain of the wood and if you feel it with your fingers, it almost always feels rough. Subsequent coats exaggerate the flaws. I feel sanding after all coats is necessary to remove attached dust and give you a smooth platform for the next coat. 220 is fine for the first couple of coats, but I like the ultra smooth finish 400 gives. That may just be personal preference. I have finished hundreds of cabinets, doors, bars, etc and found this technique to work well. – shirlock homes Mar 14 '14 at 7:53
I tack clothed well before the first coat and even hit it with some mineral spirits and let dry before starting. I didn't notice any difference between the areas I did with sanding between coats (because they had a 14 hr dry) and the 4 hr dry and no sand, in both cases the second coat smoothed out the raised grain. I think the key is to do it while the first coat is still soft enough to allow the next layer to easily ad hear to it. – virtualxtc Mar 16 '14 at 10:02

If I may add something here: they sell polyurethane that is pre-thinned. One brand I've used is Min-wax Wipe-On Poly. It's intended to me wiped on with a rag and I've never had any issue with bubbles.

This is actually all I use any more. I have not examined the cost though; it could be that this product costs more than simply buying poly along with a can of thinner.

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It is more cost effective to thin your own, but you are absolutely correct that wipe-on poly is just thinned poly. – Ethereal Mar 13 '14 at 14:43

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