Thinning Polyurethane for Spraying

Many different solvents can be used to thin poly, but the resulting slow-drying overspray can turn into a serious problem. September 14, 2006

I have a customer who has specifically requested a sprayed polyurethane finish. I use a conventional Spartan spray gun, no HVLP or LVLP yet. The finish is a bit too rich to atomize properly. What can I use to thin the poly to get a good laying finish?

Forum Responses
(Finishing Forum)
From contributor C:
You can use mineral spirits, naptha, lacquer thinner... assuming that you have an oil base poly (not a waterborne). Naptha and mineral spirits are safer choices, but I often use lacquer thinner and even combinations of any of these products. Mineral spirits are the slowest drying and lacquer is the fastest of these three. If you want something really slow, you can use kerosene.

From contributor M:
Be very careful if you decide to use kerosene, as it is very slow drying. Only add a few drops. As they used to say, a little dab will do you. First try the mineral spirits, and then check the dry time. Adjustments can be made depending on the size of the pieces you're working on.

From contributor D:
This is really a very bad idea. The overspray will be wet on everything. Spraying polyurethane is 500 times worse than spraying precat lacquer or conversion varnish, as the stuff takes forever to dry. This just isn't a problem on the article being sprayed, it's more of a problem with respect to the spraying area. You really don't want to do this, trust me.

From contributor M:
A few drops of kerosene would have very little effect not only on the overspray, but very little effect on the flow out and the dry time, although it would change them a little. This is also true with adding a few drops of this distillate to an oil glaze; if you increase the kerosene in either the polyurethane or the oil glaze, it will take forever to dry. It's the amount you add that makes the difference.

From contributor T:
Be aware that you will have a sticky mess all over your exhaust ducts and blower fan that will eventually be a pain to scrape off. Filters don't stop all of it. Fast drying finishes just blow away; slow drying finishes stick and become tough. I would talk them into using a waterbased urethane finish like Fuhr 255 if it were my client. Look into a w.b. polyurethane. Tell them Varathane is for brushing.

From contributor W:
I sprayed varnish.... once. It should be brushed or thinned way down and wiped. I wonder why anyone would want to slow it down?

From contributor M:
There are many companies today that spray poly and have been doing it for years. Like any other coating, there is a learning curve, and the proper solvent reduction and the gun adjustments are two of the things that need to be worked out. Normally, poly is slow drying, and lacquer thinners are added to speed it up, but there are many different formulas. Just as most other coatings have additives, so does polyurethane, and lacquer thinners and kerosene do work in poly in moderation.

From contributor C:
It is true that the overspray from slower drying products like poly is much more troublesome than that from the catalyzed finishes and lacquers (which is usually dry before it lands on anything). Since I have switched to HVLP, I have not found this to be very annoying, but before, when I used a standard spray gun, it used to coat everything in the shop. I use it rarely now and in moderate amounts and have few problems. The overspray is reduced by about 90 percent with my HVLP vs a standard gun. You may want to pick a nice day and spray this project outside and then bring the pieces in for drying. Watch out for bugs, though... I just hate it when a giant fly goes swimming in a freshly coated door or a huge herd of gnats descends like locusts to devour my livelihood.

From contributor R:
Overspray is the least of your worries. With an open time (before dust free) of an hour or more, any dust, gnats, flies, sawdust, stupid carpenters, etc. can ruin your finish. I don't know about you, but I work in a cabinet shop and dust is everywhere. Even though I have a closed off area with a pressurized room, I still wouldn't want to use poly on cabinets. The other thing is problems trying to glaze or shade. I would tell the client that it's a bad idea. Do some samples with a product you're comfortable with and show them to the client. They usually will respect your opinion as a professional.

From contributor W:
Gotta get up early to keep up with contributor M - there certainly are a bunch of poly(somethings) out there and most are easy to spray. Because the "finish was a bit too rich," I have assumed it is a non-catalyzed oil based finish intended to be brushed (so, naptha, MS, etc. are the solvents to use). If it is, it polymerizes by oxidation - a sloooow process, not by solvent evaporation - a faster process, and not by a catalyzed reaction - a very fast process. So it's going to stay wet for a long time (as has been stated). Now we're spraying this from a conventional gun (transfer efficiency 35% or less). That means that even with your gun perfectly dialed in, 65% of this wet finish spray is going to go into the air as overspray and it's going to come down somewhere and it's going to be wet when it comes down and stick to whatever it comes down on. So even if you spray it in the garage on a windless day and have eliminated all possible airborne contaminants, it would be a good idea to cover the bikes and lawnmower.

Aside from that sage advice, I would recommend that you get a waterbased poly (acrylic or urethane or both) and rent a little turbine HVLP to lay it down. You'll need half as much finish, it'll dry before the bugs or dust can get to it, your recoat time will be much shorter, and you'll be able to sweep up any over spray. Or if you can handle it, a CV would be even better.