Troubleshooting a "Mottled" Look with Solventborne Spray Stains
Advice on tip size, pressure and flow settings, and spray technique to achieve even application of stain. June 30, 2009
Question
I recently purchased a pressure pot setup and started using spray-only solvent-based stains. Having sprayed two kitchens with some success, I have a couple questions with regards to fluid/pressure settings. I am using a Devilbiss CVI high transfer efficiency gun with 1.0 fluid tip, with 5 psi in the pot and about 20 psi of atomizing air. Finish is roughly same viscosity as lacquer thinner.
My problem is that I am getting a mottled finish - with areas of the fan being randomly lighter or darker than others. I suspect that this is because I need to move the gun so quickly, due to my heavy fluid settings. I am wondering if I need a smaller fluid tip, because when I close down needle further - the finish starts to spray from tip (adjusting with atomizing air off).
Would more pressure in the pot allow me to turn down fluid needle further while maintaining a nice stream of fluid? Should I be concerned if the fluid is spraying/squirting out tip, as opposed to a nice stream (atomizing air will be able to still evenly spread out finish)?
Forum Responses
(Finishing Forum)
From contributor Y:
I think that the 1.0 tip is ok. You should have 30-35 psi pressure in the gun. Then it’s all about adjustment of the gun. Wider fans sometime help. I spray ultra penetrating stains and they are really thin as thinners and I don't have any problems, although I'm using cup gun. Once again, it’s all about gun adjustment.
From contributor R:
You can also thin out your stain so you can spray a little wetter and get your color in a couple of passes. That will make the color much more even and allow you to get into tough areas like raised panels.
From contributor J:
Are you using the HVLP aircap or the transtech? I have the same gun and find with the HVLP aircap I get sort of a mottled look, almost like blotching. I now only spray with the transtech cap, and as suggested thin the stain to build the color slowly instead of trying to achieve it in one pass. I also use about 5psi in the pot when spraying lacquers, but about three psi when spraying stains.
From the original questioner:
I will try knocking down the pot to 3 psi. I assume you are using a 1.0 needle and tip, not 1.4, right? I love the gun, it does an amazing job with my post cat lacquer. I do a vertical and horizontal pass so that I end up with wet coat. Thought mottled look was due to air turbulence with moving gun at fast speed. Do you check the fluid pattern before turning on the air? Good suggestion with multiple thinner coats. The stain I use is actually wiping, thinned 50/50 with xylene. It gives a pretty nice finish and allows me to apply single wet coat, as opposed to multiple light dry coats.
From contributor J:
Yes, 1.0 tip, pressure as low as possible where it still atomizes well, and pot pressure low so as not to get a huge volume of material. I find slow and steady is the way to go with the spray stains. I use the Transtech for stains and the HVLP for clears. By using a wiping stain as a spray stain, I think you may be looking at adhesion problems with your sealer and topcoats. Wiping stains are meant to be wiped as they contain little to no binder in them. Look into the MLC Amazing stain, Chemcraft spray stain, and I think there are a few others that are spray only.
From the original questioner:
These are custom stains in Becker Acroma line (called BAC). I'm not sure what the binder is but they are a combination of alcohol dyes and pigments (from concentrated paste). I have had no adhesion problems. They do have clear stain base (used with wiping), but think they mix more Xylene for faster drying. It’s really new to me and hard to get clear story from the coating supplier. It’s nice to be able to send dry sample and get back custom-matched stain in ready to spray consistency.
From contributor B:
The proper way to set up pressures for pressure feed gun are as follow's. First start with air on to the tank but with both fluid pressure and atomizing air off. You want a full trigger pull at this point. Now pull the trigger and slowly turn the fluid pressure up until you have a stream coming out the gun about 506" and then falls off. This is a starting point. Now slowly turn up your air until you atomise your material to your satisfaction. Too high of an atomizing pressure will cause undo overspray and can also cause the pattern to split.
Pressure from here can be adjusted for the speed that you want to go. Most guys I see spraying stain end up around 2-3 of fluid pressure (sometimes even lower) and 25-30# with the 507 air cap (HVLP) and around 35# with the Transtech 513. The 513 does work best but does have a little more over spray. Also with stain be careful how close you get your gun to your work. Your gun should be 7-8" away from you work. Remember always spray at the lowest possible pressure to get the work done. Numbers on the gauge should only be used as a reference point as these numbers can change with different conditions (temperature and viscosity can make a big difference day to day).
From contributor S:
On the BAC spray stain base the binder is Vinyl, however if this is a custom blend by your distributor the type and the balance of pigment verses dyes and the percentages play a huge role in the process. The settings you mention 5lbs in and 20-25 atomization sound good to me.
From contributor J:
All great suggestions, however I think it would be worth it to look into a smaller nozzle/needle if possible, as well as a smaller aircap to break up the stain better. Typically a 0.8, 0.7 is where you should be. I'd also caution against the wiping stain. If you don't wipe it - you will have a problem at some point.
From contributor B:
Interesting problem you are having. You are using a very popular gun that has a great track record for spraying low viscosity spray stains. The only other gun I would recommend would be a Sata 1000 or a Sata 3000. This gun does a slightly better job atomizing the material and laying out a uniform pattern. You may want to try a different spray stain. The most effective spray stains are made with microlithe pigment and not your conventional pigments (such as 844 and 866 industrial pigments). Many paint companies tend to use standard industrial pigments because they are available and less expensive then microlithe pigments. Microlithe pigments have become more popular the last couple of years because of their ability to bring out the natural beauty of wood. You will be amazed on how different material affects the final outcome.
