Guerrilla Guide to Spray Finishing
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It’s fast, forgiving and affordable – and you don’t need a paint booth.
Tthe most difficult part of learning to spray finishes is to convince yourself that you can learn to spray finishes.
Of all the ways to apply the finishes – brush, rag, rubber and spray – there is only one simpler method of spraying: rub on the oil. Spray finishing is always faster than any other application method, surprisingly. I can spray two coats of lacquer on a chest of drawers and assemble it in a few hours (compared to a few days with a finish applied by hand).
Oh and one more thing: in my opinion, spraying produces better results.
Plus, with good misting systems that cost almost as much as a half-decent cordless drill, you’ve just run out of reasons to avoid spraying.
If you are a professional trimmer or trimmer, this story is not for you. Most of the resources I’ve read about finishing are aimed at people who work in furniture stores or in industry. This article is for the home carpenter who wants to get the speed and quality of a spray finish but has no money or space for a dedicated configuration.
I first learned to use a spray gun as a teenager working in a door factory, and I have used everything from industrial equipment to do-it-yourself plastic units. And while I could spend most of this article discussing all the differences between the systems – high pressure units versus high volume and low pressure units (HVLP), purge versus purge guns – I don’t think it is as important as the techniques of base involved in spraying. (I think most domestic carpenters would be better off with an HVLP system, but that’s just an opinion.)
All systems are more similar than they are different: the air is forced through the finishing material, atomising it into a fine spray that you control with a spray gun. You control the mix of air with the finishing material, the shape of the finishing cone that splashes from the gun and (obviously) when the finish comes out of the gun and when it isn’t.
These are easy things to learn to control.
The second part of the spray finish that confuses people is how to actually spray the job. Where do you start and end? How do you finish the interior spaces? What about the legs?
I will discuss all these aspects of the spray finish. But first, let’s talk about what you can and can’t spray.
What to spray
Theoretically, you can spray almost any finish if you can get enough air to atomize. But for a practical do-it-yourself carpenter at home, it is best to spray finer finishing materials than latex paint.
Finishes such as shellac, lacquer, dye and varnish for milk are all easily sprayed even by the simplest spraying systems. You can in fact dilute heavier paints and finishes so that they can spray. But I rarely do this because the lacquer and shellac dry faster and spread evenly, so that spraying a slow-drying heavy paint is silly in many (but not all) cases.
When you buy a spray system, the more expensive units have more power and can atomize heavier finishes – that’s what you’re paying for (more usually a nicer spray gun). If you plan to spray only light finishes at home, you probably don’t need a heavy system.
No matter what equipment you purchase, you will need some matching needles and fluid tips in different sizes for the spray gun. If a small tip does not atomize your finish, switch to a larger tip (or thin the finish). Having some common dimensions will help avoid frustrating dead ends.
Some systems have “viscosity cups” that allow you to see if the finish is thin enough to spray. I generally don’t joke with these. If the finish is thinner than pancake syrup, it will probably work well.
Where to spray?
For most of my career I have sprayed in an industrial paint booth. These are nice, but I don’t think they are practical for the home carpenter. You could equip a ventilation system with a fan and special curtains, but this is a nuisance for some carpenters and can bring a lot of solvent smells into the house.
At home, I spray outside and with a respirator. Yes, sometimes I have to wait for a decent day, but I was surprised at how I can even spray in the Midwest in December by spraying a piece of furniture and then bringing it to my heated store to cure. Really, there are only two months (January and February) when I don’t spray.
Install the gun
So now that we’ve ordered the spray booth, let’s install the gun. All the guns I have used allow you to check how much finish ends up in the air flow; some systems also allow you to increase or decrease the air flow, both from the gun and from the compressed air source.
If you buy an HVLP system, you will probably only be able to adjust the amount of finishing material. So setting up the gun is easy. Start with fluid control fully lowered. Pull the trigger and start opening the fluid control. You will see the fan getting bigger and denser as it fills with atomized finish. When you reach the point where the fluid nozzle opens and the fan does not change in size or density, stop.
It’s a good mix to start working with on a test board.
If you have a system that also allows you to control the air flow, I recommend that you read the instructions for your system. It will tell you a good starting point for regulating the air flow. Balancing the airflow and finishing material requires some experience and it is a good idea to use someone else’s gun that has been installed so you know what a good fan looks like.
The other important adjustment on the gun is the air cap: check if the fan is spraying a horizontal finishing band, a vertical one or something rounded.
A horizontal fan is ideal for spraying vertical components such as legs or spindles. A vertical fan is ideal for panels. I like a circular motif to work inside a crate.
Oh, and you have to learn to use the trigger. It’s a good idea to release the trigger when your fan doesn’t hit the job, even if it’s only for a second. This saves finishing material. It also helps you not to get too much finishing material on the perimeter of a panel.
How to spray a board
So with the gun settings somewhat compounded, it’s time to spray a test card to see how the finish is finishing, if the fan is too wet or dry. Some people use cardboard or masonite for this step. I use a piece of scrap that is the same species that I am spraying and that has been prepared in the same way as my project. Raw or worked timber won’t tell you what your gun is really doing. The cardboard absorbs the finish too quickly and is rough.
When spraying the board, keep the gun approximately 10 ″ away from work. Start by spraying the bottom of the board and start at its back edge. You want to work in a robotic way: keep the gun at 90 ° at work and a constant distance at all times. Press the trigger a moment before the fan hits the card and release the trigger a moment after the fan turns off the card.
After spraying the back edge, spray the fine-grained edges of the board in the same way. Then position yourself to spray the front edge. Spray the front edge. But instead of stopping after the front edge, adjust your wrist and continue spraying the face of the board.
Your traits must overlap: I overlap a third of my previous stretch to the next stretch. The board will change color as you spray the finish, to help guide your efforts in the round of your shots.
After spraying the board, crouch down so you can see the reflection of the film lying on the board. It won’t look perfect (usually) but it should appear constantly wet with no dry areas or huge puddles. If it’s dry in one area, you’ve probably lost a spot. If it’s dry streaked on the board, then you haven’t overlapped your shots or you have to open the fluid control a little.
If you have puddles (or lakes), or you linger too long in one place or you have too much liquid coming out of the tip.
Allow the finish to be set (for lacquer or shellac it may take 5 to 20 minutes). Then evaluate the test card. It could be a little rough, like your skin after a day of no shaving. Sanding down will fix those nibs. The important thing is that the film is subtle and coherent. If it looks dense and opaque, it is necessary to reduce the fluid in the fan. If it feels like a rough sandpaper and there is no film, you need to increase the fluid in the fan (or you have kept the gun too far from work).
If you are not sure of your results, wait for the finish to harden again (about 30 minutes in a warm room). Then use an abrasive sponge no. 300 to remove the fuzzy nibs and level the first coat. If you get white powder when sanding lacquer or shellac, don’t worry. Thats good. If you don’t get white powder, let the finish harden longer and try again.
Then repeat the spraying process for the test card and evaluate the results. After spraying a few projects, you’ll immediately dial in the gun settings, especially with a familiar finishing material.
But not everything is advice
Spraying complex assemblies can seem overwhelming. Helps break down the project into internal surfaces, secondary surfaces and primary surfaces.
Internal surfaces – like the inside of a chest of drawers – should not be sprayed at all. If they get a little overspray, nothing serious. Historically, the interior of the cases has remained unfinished. This philosophy can also extend inside the drawers and wardrobes, but this is an aesthetic choice.
The secondary surfaces – shelves, interior of the doors, backs of the lockers in the open boxes and so on – need finishing, but they must not be as perfect as the primary surfaces. The primary surfaces are the striking points: the table top, the sides of the case, the fronts of the drawers.
In my shop, I don’t spray interior surfaces. So I start by spraying all the secondary surfaces. The final surfaces to be sprayed during a coating session are the primary surfaces.
So when spraying a library, for example, I:
1. Do not spray the back of the scoreboards.
2. I set the air cap to a diagonal to produce a round cone and spray the inside of the case and shelves.
3. Set the air cap to create a horizontal fan and spray the front edges of the case and shelves.
4. Spray the sides of the case.
5. If the case has a visible top, I set the air cap to create a vertical fan and spray the top.
Another good strategy when spraying is to spray as many parts as possible that can be stretched out as much as possible. If you can pull out the shelves and spray them on trestles, you will get better results than spray them inside the carcass.
After setting a finishing coat, level with an abrasive sponge (or steared sandpaper) and add a second coat. For most projects, two coats of varnish or shellac are many. Three hands are fine. After three hands things will start to look plastic.
Final tip for leveling the finish: if it looks good but still feels a little rough, wait two weeks and rub the coating with a brown paper bag. It is coarse enough to level the nibs without showing visible scratches.
Spray equipment must be clean to function. Most of the problems I encountered with spraying are caused by poor gun hygiene.
When you’re done spraying, run a few ounces of solvent through the gun. Shake the gun occasionally while spraying the solvent to make sure it cleans the entire cup holding the finish.
Disassemble the gun and immerse the needle, the tip of the fluid, the air cap and any other part that came into contact with the finish in a little solvent overnight. Lubricate the gun needle and trigger (most people use petroleum jelly or mineral oil; do not use silicone).
If all this seems simplistic, it is because it is. Yes, there are positive points in the spray finishes, especially when entering advanced techniques, such as toning a board. But if you just want to apply a finishing finish and make it look perfect after a few hours of work, the spray finish is definitely something to try.
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