Home  Why  Business  Woodworking  About us  Contact us
 

Spraying a finish
and other application techniques

Application techniques

Oiling

The BLO, Tung Oil, or Oil mixture is applied by brush or rag. The key is that anything that doesn't soak into the wood in 5-10 minutes should be rubbed off before it dries. Many oils give off heat as they dry, so be sure that you don't leave oily rags in a pile where the heat can build up... it can start a fire. (I know a building where the painters spread out their oily rags and left. The contractor "cleaned up" after the painters, sweeping their rags into a pile - and that night the building burned down.)

brushing

Use a good brush and technique... don't expect a great finish by casually spreading the finish with a $1 disposable brush. The material must dry slowly enough and be thin enough so that it can flow - level but not drool - to eliminate brush marks before it dries. It must be thick enough to build the finish.

wiping

You can buy a special wipe-on finish, or can thin most regular oil-based finishes (perhaps 50% paint thinner). The finish is wiped on with a rag or sponge brush. Each coat is so thin that there are no brush strokes. Each coat dries fairly quickly, but many coats are required to build the film thickness to protect the furniture. Same story as oiling - spread out the wiping rags!

padding

This is a special technique that mixes multiple finishes to lubricate the pad that is used to wipe on the finish. Special pad (called a tampon), special technique, special mixture of finishes, time consuming to learn and do, but great results. French Polish is padding shellac in combination with other materials.

spraying

Once you learn to spray, you won't go back. But it does require skill. One instructor said, "If you have an air compressor, buy a "conversion" gun and a gallon of lacquer, some lacquer thinner, and for $150, you will learn to spray and have all the equipment you need." It does require skill - I have had a spray outfit for over 40 years, and am still learning. It is definitely the way to go.

Spraying tools

If you just want a quick opinion of what type of spray outfit to buy, look here.

Conventional gun

The "old fashioned" conventional spray guns typically used 40 psi or more compressed air at the gun. With the high volume of high pressure air, a lot of paint was atomized (made into a mist) but a substantial percentage never made it to the workpiece. Estimates of the amount of lost finish were as high as 50% or more. These guns now cost as little as $25 but are obsolete for practically any purpose.

HVLP gun

Careful design of the air path and nozzle allowed the use of a much lower air pressure at the tip of the gun - one legal definition said the tip pressure had to be below 5 psi to be called HVLP, as required in some states. A traditional HVLP gun is run by a turbine rather than an air compressor - a unit that sounds like a backwards vacuum cleaner, pushing a large volume of relatively low pressure air through a relatively large air hose. These systems (turbine and gun) typically cost $500-800 although there are some cheap versions available as low as $100.

Conversion gun

A conversion gun is an HVLP gun that looks like a regular spray gun, that runs from an air compressor rather than a turbine. Boyle's law says that the volume and pressure of gas are inversely proportional (all else being equal), so if the volume of air doubles the pressure drops in half. We may put 80 psi in a small air hose, and reduce it to 20 psi going into an HVLP gun, and as the air expands inside the gun on the way to the tip, the volume expands and the pressure drops to 5 psi or less. These "conversion" guns typically cost $70-$300 or more. These guns often require 8-10 cfm of air, but if you have a good size tank on your compressor (at least 10 gallons), and use a non-bleeder gun (see below), you can probably use a compressor that provides 4-5 cfm. The trigger on the gun will be pulled half of the time or less, so the compressor can also put air into the tank during the instants you are not spraying. (The compressor may work very hard if you are a fast sprayer.)

Bleeder - non bleeder guns

Some guns are bleeder, others are non-bleeder - does this require first aid? A bleeder gun is one that uses air continuously... the trigger/valve only controls the fluid. In theory my turbine can overheat if the air is blocked, so my HVLP gun is a bleeder gun (although I suspect it is a bleeder so the gun doesn't need to include an air valve). Some worry that the continuous airflow can stir up dust, but I take advantage of it to clear away dust. A non-bleeder gun has two valves controlled by the single trigger, one for air (on first, off last), and a separate valve for the material being sprayed. If you are spraying from a compressor, be sure you use a non-bleeder gun.

Airless Sprayer

An airless sprayer puts extremely high pressure on the paint, and doesn't use air at all. An HVLP system can theoretically be used for latex paints, and an airless sprayer can theoretically be used for a fine furniture finish, but I have found it worth the investment to have separate systems for each purpose - an airless sprayer for latex, and an HVLP system for finishing furniture. A safety note about airless sprayers... the paint pressure is so high, the paint will penetrate your skin if you accidentally spray yourself. In extreme cases, amputation is required.

Where's the meat? (okay, where's the finish material)

Gravity feed

Some guns have a paint cup above the gun, so that the paint feeds by gravity. Some people argue that it makes the gun hard to hold (I don't agree). It has a very short paint path to clean (and thus a very small area to clog or cause troubles). It is easy to use "all" the finish in the gun.

Siphon feed

As air is blown across an opening it creates a vacuum (venturi effect). So as the air goes out the gun, it can suck (siphon) the finish from a cup below the gun. The cup is closed enough to not spill the finish but is not sealed... there has to be an opening for air to get in. My old conventional guns were siphon feed - they worked okay, but the slightest plug to the air hole or feed tube and the paint volume would drop. I found them a little "fussy." There was also a long tube to clean, and it was hard to use "all" the paint in the cup.

Pressure feed

A small amount of air from the gun can be diverted to the paint cup below the gun. The cup is sealed, so the paint is forced up the tube and into the gun. It works pretty well, but you have to keep the seal clean on the paint cup, and like the siphon feed, you have a relatively long paint path to clean, and it is hard to spray all the paint in the cup.

Pressure pot

If you can pump the paint from a cup below the gun, why do you have to hold that cup (or pint) of paint while you are spraying? Why not pump it through a separate tube from a pressurized paint pot? If you will be using the same finish "all day" this is a great idea. Some people use a small (1-2 quart) pressure pot that they keep next to them while they are working (some even wear it on a belt). I have a 5 gallon paint pot (that would hold a gallon can without emptying the can) from years ago that I would take up a ladder while painting a house, but I learned that the paint pressure (volume) changed, if the gun was higher or lower than the pot. And the material hose, from the paint pot to the gun, was a monster to keep clean. I don't use it any more.

Where's the air?

Big Compressor

Some day I will grow up and have a big air compressor in my shop, with air outlets around like electrical outlets. Someday. It will probably have a 5 hp or so compressor, oil lubricated, with air filters to get rid of moisture and oil. One trick will be to have two tanks... a relatively small (5 gallon?) tank, so I can get quick air if I just need a little, without waiting for the compressor to fill the big tank, and a larger tank with a valve to "turn it off" that I would open when I will be using a lot of air, and am willing to wait for the compressor to get the large tank filled. With compressed air, the air is hot as it is compressed, but cools in the tank. Then it cools further as the pressure is reduced when it is used. Bottom line, cool air, but it may be moist.

Small Compressor

These less expensive, relatively portable units, are great - I have two. But they are less efficient, noisy, and may require me to pause as the compressor catches up with my use of air from the tank. Some of the newer "conversion" guns recognize that they may have to work with these compressors, so are more efficient with air use. Some are even called LVLP, since they use a lower volume of air than the usual HVLP conversion guns.

Turbines

Various vendors offer turbines, which sound like a vacuum cleaner, and have a series of 1 to 4 compressors run by a single motor. Could somebody invent a single compressor that does the work of four? Probably, but most of the turbines seem to use the same internal parts, so are rated by how many "stages" there are to push the air. Since lots of air atomizes the finish better, most people want a 3 or 4 stage turbine. Since the air is not compressed and stored, but just pushed, the turbine air tends to be warm and dry, rather than cold from a compressor.

How to spray

Get Ready

Spray guns have different size nozzles for different thickness paints, but few people I know have invested in multiple nozzles. Water based lacquer works well, without thinning, with the "standard" nozzle on every gun I have tried. Nitrocellulose lacquer needs to be thinned. If you have a gravity feed gun, paint will come out the nozzle when you pull the trigger without air pressure. I have found an easy test... if you can make a stream of lacquer "squirt" across a gallon can, without air pressure, the lacquer is thinned "about right." If it only "oozes" out of the nozzle, it is too thick. If you don't have a gravity feed gun, there are "cups" that have a calibrated hole in the bottom... dip them in the finish and measure the time for the cup to drain to determine the thickness.

Maybe if I bought a jumbo nozzle for my spray guns, I could do latex satisfactorily with my HVLP system. I know some other people who are as cheap and cranky as I am, who tried this and then bought an Airless Sprayer for latex, so I went that way without trying larger nozzles.

If you want to routinely spray finer material, like stain or dye, you may need to invest in a smaller nozzle.

Ready, Aim

Successful spraying requires that the gun be kept a constant distance from the work. If you are spraying a shelf or other long piece, you cannot hold your hand in the center and spray from one side to the other - the finish will not be even. You must move your hand, pointed straight at the work, and a constant distance from the work, from one end to the other. This is a very unnatural movement that you must learn if you are going to be good at spraying. Someone who can spray can move their hand from far left to far right, without thinking, while keeping it pointing directly forward. You can practice this movement without a spray gun, as long as nobody sees you. If they see you, someone may approach you offering a white jacket with long sleeves, and you may see their ambulance in the distance.

I learned to spray with oil based paints, and the recommended distance from the gun to the work was about 12-14 inches or more. With fast drying lacquer, I find the gun should be 6-8 inches from the work - the lacquer overspray will be dry before it hits the ground. The expensive guns have a very even spray pattern, so little overlap is required. Cheaper guns create a "fan" of finish that is thinner at the edges, so more overlap is required.

When spraying, you must stop the flow of material at the end of each stroke, before you stop moving the gun. If not the material will get thicker and thicker as you slow down, stop, reverse, and accelerate...you will get a drool. This is another unnatural movement that you must learn.

If you are finishing a cabinet or bookcase, leave the back off until it is finished... it is far easier to spray a cabinet without a back, and to spray a back separately.

Most guns have a nozzle that can be rotated. If the "knobs" are at the top and bottom, the fan of finish is horizontal, ready to move the gun up and down. If the knobs are at the left and right, the fan is vertical. I glance at the gun, and know it is ready to move in the direction of the knobs. One instructor said to leave the nozzle alone and tip the gun. It works for him, but not for me.

Ready, Aim, Fire

The amount of material on your work depends on the distance the gun is from the work, the speed you move the gun, the angle you hold the gun (is the fan of paint covering a wide area or is it being concentrated in a narrower area), the volume of paint (adjusted by a knob on the gun or the pressure on the paint pot), the thickness of the paint (thinning and temperature), the air pressure (an adjustment on the gun and several other places). Bottom line, you have to try it, but keep in mind that all these factors can change. As you are learning, remember that multiple thin coats of lacquer burn in and become a single coat, so do more thin coats until you get experience.

Finishing in a corner seems like it would be easy... how can you miss? Answer, all the air that goes into the corner has to come out, so the air leaving may blow all the paint out of a corner, leaving it bare. Try coming into the corner from each side. Try drastically reducing the air pressure/volume. If your gun has the option for a round spray pattern rather than a "fan" the round pattern may work better in corners.

Finishing an outside edge seems like it should be easy. A sharp edge will direct the air and material away from the edge, and little may stick. Like the corner, you may need to do each side of the edge. I also find it far easier if I have sanded a sharp corner slightly - rounding it a tiny amount makes finishing far easier.

Practice

If you are just learning to spray, I suggest that you spray water on a cardboard box or the side of a house. Gallons and gallons of water. Hours of spraying. You can tell when you have an even coat, because the house or box will dry evenly. Water is thinner than most finises, but if you keep the water from drooling, you probably won't have trouble with the the finish. Set goals... painting up to the edge and stopping... making a line where you have to develop the skill to "stop the paint and change direction without getting a drool." It is boring, especially if you have a new sprayer and a project waiting, but it is time well spent.

Good luck! Your comments and feedback are appreciated.

Back to the index of woodworking tips
Back to the introduction to finishing
Back to the page on sheen and rubbing out the finish
Back to the page on lacquer
On to the page on choosing a spray gun