Basics of spray finishing

Start to finish with high-volume, low-pressure spraying

By Steve Maxwell

sray gun

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Spray guns are the fastest way to put a great finish on wood. That’s why they’re used all the time by professionals. Trouble is, home workshoppers have traditionally been shut out of spray finishing because big air compressors were too expensive to buy, old-style spray guns too messy to use and traditional finishing liquids too dangerous to make sense.

But advancements in spray equipment and finishing products now make sprayers affordable for workshoppers who are willing to adjust their processes in exchange for the ultimate in fast, smooth results.

None of this would be possible if it weren’t for the advantages offered by high-­volume, low-pressure (HVLP) spray systems. In­stead of a large, loud air compressor delivering small volumes of high-pressure air to a conventional spray gun, HVLP systems operate differently. A small, compact electric turbine pumps high volumes of air (at about 4 to 5 psi) into a spray gun designed especially for the job.

All else being equal, it’s easier to succeed with HVLP than with a conventional, compressor-driven gun. So, what’s the difference? Be­cause the flow of air out of a HVLP gun is gentler, a great­er portion of the spray liquid is successfully delivered to the wood surface. About 80 per cent lands where you want it, compared with 40 per cent or less for high-pressure guns. This saves money, but, more importantly, it makes HVLP much cleaner to operate. Less overspray means clean­er air. Not perfectly clean, but good enough to use in ventilated home workshops.

The quality of HVLP air is better too. It tends to be warm and dry, which is ideal for propelling all finishing liquids. Compressor-delivered air, on the other hand, is often cold and damp, contaminating finishing liquids with condensed water that can cause cloudiness and sticky results.

Water-based urethane is the second advancement that brings spray finishing onto the radar screen for home workshoppers. Al­though it doesn’t eliminate the need for ventilation, water-based urethanes are much safer than solvent-based formulations and ev­ery bit as effective when you understand the tricks for success.

I’ve sprayed more than a dozen brands of water-based urethanes, and to make them work, you need three things: the right kind of space, the right light and the right technique.

Although water-based urethanes are non-toxic, you still need ventilation and a mask. You also need to protect areas of your shop from the fine, white dust that settles as half-dry overspray around your work zone. Use plastic drop sheets or spray outdoors.

Applying too much finishing liquid is the easiest mistake to make with any kind of spray finishing. The second-easiest mistake is not applying enough. To combat both of these hazards, you need help from the right kind of light-bright, white and shining across your work surface at a shallow angle. Overhead light is not ideal because it doesn’t show where you’ve sprayed and how much. Instead, I use quartz-halogen floodlights that come mounted on adjustable stands. Raise, lower or rotate the angle of the lights so they reflect off the surface you’re spraying.

Even with ideal lighting, watch out for running drips. If you hesitate just a little too long with your finger pulling the spray gun’s trigger, you’ll get too much finish in one spot and the makings of an ugly run. Water-based urethanes are particularly prone to this. That’s why you should orient your work surfaces horizontally and flat whenever possible. It only takes 10 or 15 minutes of drying time before even a thick coat of water-based urethane is firm enough to resist running. Then you can rotate your project and spray another horizontal surface.

It will take you a couple of projects to get used to spray finishing, but once you have the knack, it’ll be harder than ever to go back to a paintbrush.

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