Spraying water based polyurethane with airless sprayer

Switching from solvent- to water-based finishes has some advantages, primarily reduced odor and fire hazard, as well as reduced harm to the environment. But the switch can be difficult because the application is different.

It’s true that pulling the trigger on the spray gun and covering the surface while keeping the gun moving doesn’t change much, but there are differences that cause enough problems to discourage switching unless forced to do so by the changing VOC laws.

Here are some of the problems and how to deal with them:

First, before you even begin spraying, you should be more diligent than with lacquers to strain the finish. Solid pieces of lacquer falling into the finish from the lip of the can dissolve back into the finish. But this doesn’t happen with water-based finishes. Also, water-based finishes can sometimes coagulate a little in the can. Always strain water-based finishes.

Atomizing

Water-based finishes don’t atomize as easily as lacquers, especially if the finish is cold. The best spray guns to use are airless and air-assisted airless, especially the latter. If you use a turbine HVLP, the more stages the turbine has the better. Three is minimum.

You can thin water-based finishes with water to improve atomization, but the thinning is different than with lacquer. Very little water makes a much bigger difference than lacquer thinner in lacquer. So try 5 percent or so at first and see how this works.

The problem with water is that it has a high surface tension, so it might not improve flow-out at all. It also might increase grain raising. So a better reducer is usually a glycol-ether solvent. The one commonly available is ethylene glycol monobutyl ether. It is sold under a number of names, including EB, butyl cellosolve and 2-butoxyethanol. Take a look at the ingredients on the can or at the MSDS to be sure this is what you’re getting.

Adding EB directly to the finish is usually a bad idea, however, because the solvent shocks the finish into crystallizing. It’s better to mix the solvent half-and-half with water, then add it. This usually works well and creates better flow-out. Keep the amount you add very small, however, until you figure out what works best with your specific brand of finish.

Also, keep in mind that temperature affects water-based finishes more than it does solvent finishes. For every 10-degree drop in temperature the viscosity increases by 10 percent. You might find that the best solution for your problems is just to raise the temperature in your spray area.

Other tricks you can use are to store your water-based finishes in a heated office overnight if you lower the shop temperature and to use heating pads, bucket heaters or inline material heaters.

Better prep

I see instructions to sand to a finer grit (280–400) to reduce grain raising. I haven’t experienced this and it’s a lot of work to sand to that fine a grit.

I think it’s more important to pay close attention to sanding the first coat smooth before applying the second. Once the first coat is smooth to the touch, the next coat of any finish will go on smooth as long as the finish is atomized well. It takes more work to get the first coat of water-based finish smooth, however, than it takes with solvent finishes.

Because of the grain-raising problem, some manufacturers promote their water-based stains and finishes for reduced grain raising. Be doubtful.

Drying

Though improvements are being made, water-based finishes dry slower than lacquers. Moreover, you don’t have solvents you can add to speed the drying as you do with lacquer. Adding acetone to a water-based finish only causes problems.

So the best way to speed the drying of water-based finishes, especially in humid conditions, is with airflow or heat. Airflow can be from the draw of the spray booth or from fans. Be sure the air being circulated is clean or you’ll introduce debris into the finish.

You can also use lamps to speed the drying or, of course, warm the air in the spray area.

Cleaning

One of the biggest differences switching from a lacquer to water-based finish is the increased difficulty of cleaning spray guns and equipment. Simply running water, warm water or soapy water through the gun and other equipment doesn’t work well in my experience, especially with spray guns. There’s almost always some finish left in recesses and it will harden there and be difficult to remove.

There are three possible solutions to this: The first is to disassemble the spray gun at the end of each spraying session and scrub the finish from the parts with a stiff brush.

The second is to use a mixture of water and glycol-ether solvent to spray through the gun and/or to scrub the disassembled parts.

The third is to use a special gun-cleaning solvent supplied by the finish or spray-gun manufacturer.

Another difference from lacquer is that water-based finish tends to collect on the fluid nozzle right at the tip of the needle. This happens because the water-based finish isn’t being redissolved by the thinner. You need to be aware of this buildup and clean it off whenever it occurs or it could be blown onto the finish surface and leave a bump.

Staining

Staining is more complicated with water-based finishes than with solvent finishes. If you use a water-based stain, you experience much faster drying than with oil stains. You might need two people — one applying and the other wiping off — to be successful on large surfaces.

Using an oil stain can present a bonding problem with the finish. You can let the stain dry longer or use heat to get it to dry faster. Some water-based finishes bond well over some oil stains depending on the formulations of each. Check with your supplier, then test well.

You can also use a water-based spray/no-wipe stain. The effect you get is different because of reduced pore definition, but it might work with your objectives. Spray/no-wipe stains (also called spray-to-color stains) are highly thinned so you don’t get striping. They are meant to be sprayed in three or four passes.

Tint the finish

The final tip concerns dealing with the cold appearance of most water-based finishes when compared to lacquers. Often a stain underneath takes care of this, but if you aren’t staining, you might want to add a little water-soluble dye to the finish to warm it up.

Bob Flexner is author of “Understanding Wood Finishing” and “Wood Finishing 101.”

This article originally appeared in the July 2016 issue.

Can you spray polyurethane with an airless sprayer?

Can you put water based polyurethane in a paint sprayer?

How do you thin water based polyurethane for a sprayer?

What size tip should I use to spray polyurethane?