Fundamentals of Spraying Stain

      A finisher who's accustomed to working by hand gets advice on his first attempts at spraying stain. December 30, 2005

Question
I've never sprayed stain, and have very limited experience spraying at all. I have a DeVilbiss Finishline HVLP gravity cup gun that I got about 8-10 years ago. I understand it is a pretty good gun. I am looking for any advice on setup. I will be spraying Mohawk Ultra Penetrating Stain and have both a 1.7 and 1.8 tip. From searching the archives and internet in general, it appears the starting point for setup is approx 23 psi of air at the gun, backing the fluid setting out 2.5 turns, and then adjusting from there until proper atomization. But this stain is thin like water, and I am not quite sure where to start or even what to look for when I get there. Although my small stable of hand applied finishes has always worked ok, it is too time consuming in many cases, and I'd like to add spraying to my repertoire - waterborne only. My current project just cries out for spraying this stain, not wiping - it is cork flooring, about 150 pieces of 1'× 2'.

Forum Responses
(Finishing Forum)
From contributor A:
If you are spraying a wiping stain then almost any setting on the gun to get the product on the substrate will work. I would start off with the setting you stated except I would back the fluid needle all the way so that nothing comes out. Then slowly adjust it until you get enough coming out to lightly flood the wood. Then wait the amount of time you need to let the stain soak in and wipe it down. If you were planning on spraying a no-wipe stain or dye, you would need to get a smaller needle set, probably around a 1 mm. Then you would do the same thing until you get a nice streak free coating.



From contributor B:
You have an excellent setup for spray staining. I've been doing this for years on pianos, and it works great. There are two ways to go. You can spray a heavy flow coat, wait a few minutes, and wipe off the excess. Or you can spray very thin coats and watch the drying pattern as you spray. I've never sprayed a cork substrate, so I wouldn't know which technique to use. Try both on a piece of scrap. You can also play around with the retarder Mohawk supplies if it's drying too fast. This allows for more control and also thins out the stain. The 1mm tip is a good idea, as it will also allow for more control.


From contributor C:
It depends on the base the stain is on. If it is a thinner solvent base then you have to build it to the desired color with no wiping. If it is water base, the best way is to flood it over, and wipe it off. The time you wait doesn’t matter, because wood absorption goes to zero with time. When I work at a door, I close the fluid nozzle, and then open it till the fluid covers the surface but does not dry before I can wipe it. If it dries it is going to be darker and hard to wipe to the right color. Practice on scraps and sample with different times.


From the original questioner:
I was planning on no-wipe, for the same reason I want to spray, and not wipe the stain on. Remember, this is cork, not wood, and the textured surface seems to make wiping a bit challenging. The Mohawk Ultra Penetrating Stain is actually an alcohol based dye I think. Not sure of the exact terminology, but I do know it is alcohol based, and has no pigment particles - or at least certainly not like their oil based wiping stains. The Mohawk guy said I could actually use water as a retarder. I was reading through the documentation for my gun the other day and it mentioned only a 1.7 and 1.8 tip available, and I got both. I'll do some checking to see if I can get a smaller one, or if the smaller ones are only available for the newer guns.


From Paul Snyder, forum technical advisor:
Mohawk Ultra Penetrating Stains are metallized aniline dyes, which gives them good fade resistance. They can be reduced with alcohol, acetone, or water. I like to use lacquer thinner or acetone to eliminate raised grain and speed the drying time. There's no benefit to wiping after spraying dye, and it can cause problems on some woods if you do. Some woods take dye a lot better than others. You can get these woods wet with dye and when it dries the color is great - deep and even. Other woods display some serious problems when you use dye, and spraying the dye can create its own problems. Woods like walnut and mahogany accept dyes well. You can wet the wood and either wipe the excess or just leave it to dry. Woods that will blotch when you stain them will usually blotch when you use dyes. Cherry, maple, and birch are a few that come to mind. On ring-porous woods, like oak or ash, the dye will not color the large pores the same way as the main body of the wood. The dye works well with these woods; but you have to color the pores in a second step – either a wiping stain over the dye or a glaze over a washcoat that seals the dye.

Some problems that come with spraying dye are oversaturation, flooding, bleeding, striping, and poor coloring in corners. Unlike wiping a dye, where the color reaches a limit, spraying can oversaturate the wood with color and it gets darker and darker. I usually like to spray the dye in one even wet coat though some finishers prefer to mist on multiple coats to build the color. The problem I have with multiple coats is maintaining the same color intensity and consistency over numerous surfaces. Applying the dye too heavy, flooding the surface, causes blotching, bleeding, and/or color pooling. A light coat that wets the wood and flashes off quickly keeps the color even. I have to pay close attention when spraying dye to make sure it's heavy enough to wet the wood and overlap the passes enough to avoid spraying stripes. I usually set the gun to reduce the fluid quite a bit to produce a gentle “fog” pattern and back off the surface of the wood a couple extra inches. If it's not wet enough, it produces a "hungry" look where the grain and figure are muted. When you spray into corners and recesses, the turbulence from the atomization air can keep the dye from wetting the wood evenly. Keeping the air as low as possible and good spray technique will minimize or eliminate the effect.

Some finishers like to use a washcoat on some species of wood (e.g., maple) before they spray dye. The washcoat partially seals the wood and limits the dye penetration. This makes it easier to spray the dye and avoid blotching and/or oversaturation. Make sure the washcoat is not dissolved by the thinner you use in the dye. After the washcoat dries, sand it lightly with 320 grit until smooth, remove the dust, then spray the dye. Do some tests on each wood you plan to dye to find out how it behaves - this will help you avoid the problems that can happen.



From contributor D:
I find the best way to spray no-wipe stains or dye stains is to up the air pressure and lower the fluid pressure. Choke your needle down until you get the desired look. Even though you use a HVLP setup, you will not have any wasted material by turning your fluid pressures down. I would start out with about 30 psi air and 7-10 psi fluid. Keep your fan open.


From contributor E:
Cork is a very porous material. I would suggest that then when you are making up your samples on the cork, you first apply a thin wash coat or two, and then allow that to dry, and then apply the stain. Try coming up with a formula where you pre-reduce the stain, so you only need 2-3 passes with the spray gun to get your final color.


From contributor F:
You are spraying a material that has a very low viscosity. You said yourself that it is like water. Your needle/nozzle selection is way too big for such a thin viscosity. Load the smallest needle/nozzle (they are mated and they have to match or you can ruin both) into the gun and follow contributor D’s advice on optimizing your gun for this application. Then work out a schedule that follows contributor E’s advice on how to deal with cork. I also prefer Paul Snyder’s method of not flooding on and wiping off a dye coat. You spray it on (not dust it) like any lightly sprayed application. It will dry as Paul suggests and you will get even coloring. But you need to mimic what a robotic sprayer would be doing, spraying the same speed, distance, striping pattern, etc. That's what we do - operate as consistently as possible in our spraying movements.


From the original questioner:
To contributor D and contributor F: This is a gravity cup HVLP gun. I'm afraid I don't understand the phrase "fluid pressure" in this context. To contributor E and contributor F: I am familiar with washcoats. I typically use a 1 or 2 pound cut dewaxed shellac, or Bullseye Seal Coat. Is this sufficiently close to what you are referring to, or did you have something else in mind? I had considered a washcoat, but decided against it because this is cork flooring. Cork is pretty flexible, and I was concerned that a shellac washcoat might crack or craze under foot traffic - with all the flexing. Heaven forbid if some friend of the wife should set foot on the boat in high heels. The final finish for the floor is a urethane (not regular polyurethane) which is quite flexible. Are my fears unfounded? Would a washcoat still be applicable in this application?

I just found out that the smallest tip I can get for my gun is a 1.4. However it has to be special ordered and country warehouse is out of stock. They thought 3-4 weeks to get it (here in Vancouver BC). Don't think I can wait that long. The rep thought the 1.6 (what I mistakenly called a 1.7 earlier) throttled way down, almost closed would work ok.

I just re-thought what I wrote - shellac would probably not work here because that stain (dye) would dissolve it.



From contributor D:
Just keep your air pressure up and your fluid nozzle closed. Start spraying on a white piece of paper opening your nozzle until you reach your desired color.

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