I produced a video a few months ago demonstrating how to apply a water-based Polycrylic clear coat finish with a HVLP spray gun, an operation often touted as “can’t be done”. In this article, I’d like to revisit that subject. As with any production woodshop, finishing is a huge part of the success of a client’s project. There are myriad options to choose from, and just as many varied stipulations to be aware of for each method. Polycrylic (acrylic urethane) offers many advantages over a polyurethane (polyester urethane).
Polycrylic is water-based instead of solvent-based, so it is non-flammable and non-toxic. This does NOT mean that one should skip the respirator when spraying Polycrylic. I, for one, do not want acrylic particulates in my lungs, even if the suspension material is water-based. Also, clean up is a breeze with just simple soap and water. Polycrylic dries exceptionally fast (2-hour recoat time, even faster when sprayed) and at least 3-times faster than polyurethane (6-8 hour recoat time).
The upside is also the downside of Polycrylic, in that it dries really fast. The acrylic urethane gels quickly during application, where any more than 2 or 3 brush strokes will leave obvious brush marks in the finish. This is another reason to apply via a sprayer. Polycrylic is also a bit more expensive than its polyester cousin.
In my video I misspoke in saying that Polycrylic cannot be thinned with anything. I meant to say that Polycrylic cannot be thinned with any chemical thinner. But it does thin quite nicely with plain water. In our shop, we’ve experimented with 10-20% thinning with water and have found a seeming sweet spot at about 15%. Some ratios to help hit the percentages:
|Ratio (water : finish)||% Thinning|
|1 : 4||20%|
|1 : 5||16.67%|
|1 : 6||14.3%|
|1 : 7||12.5%|
|1 : 8||11.1%|
|1 : 9||10%|
Unthinned, Polycrylic is somewhat tricky to get to spray properly. The denser particles require as slow an airstream as your gun can cogently produce. Any serious speed causes the acrylic particles to ‘bounce’ instead of adhere to the surface, effectively wasting large quantities of finish out into the atmosphere. With proper thinning, you can run your HVLP spray gun just as you would any other finish, and achieve great results.
Mike @ Doobly-Do Wood Works firstname.lastname@example.org