Retarder to correct overspray?
Generally speaking, I would not use retarder to cure this problem.
Too much retarder in the mix can result in a finish that takes much longer to cure than it should, or that may not cure properly at all. Adding a little retarder is not going to hurt, but it probably won't fix your problem and it is tempting to push your luck and add more retarder.
I use retarder only when humidity is very high, which causes moisture blush in the finish. And then I use it sparingly. If a small amount doesn't work, I don't spray. I've had more than a few jobs that had to be done over because too much retarder in the finish prevented proper curing.
I would use the manufacturer's recommended thinner and if there is a choice between a slow and a fast thinner then choose the slower one. I'd look at other factors causing excess overspray before adding retarder. Thinning the material a bit more may help. Also check your air/material gun settings, and consider an application routine that helps minimize overspray.
Typically, cabinet interiors are the most troublesome for overspray and you cannot entirely beat this. A good and fast routine is the answer. Sometimes, I turn the gun atomizing air pressure down when doing an interior. This will result in a coarser spray (orange peel), but that is usually of little consequence on an interior surface. Trace overspray is pretty easily removed from the final coat with a light rub of 4/0 steel wool.
Some manufacturers offer other additives that are supposed to work specifically on reducing overspray, but the ones I tried have not been of any help to me.
Again, my comments are of a general nature. I have no experience with the material you're using, or the gun.
Retarder will help solve this problem provided you use the correct one.
Though I'm also unfamiliar with the material or gun, I'm assuming the Magnalac is a nitrocellulose lacquer based material. Methyl amyl ketone (MAK) is a good lacquer retarder that will not cause any problems with the durability of the coating. It will not soften the coating as other solvents sometimes will.
I recommend using MAK in combination with the correct reducer or thinner to reduce the viscosity of the material, and some MAK to slow the drying time. When added in the correct proportions it will almost eliminate all your overspray problems.
From Kremlin Technical Services:
Since there are many different coatings out there, we leave the recommendations on retarder to the experts.
However, on dry spray using the Airmix system, we recommend spraying at the lowest possible atomization pressure. We recommend lowering the air pressure until you start seeing some tails on your pattern, and/or the quality of the spray drops below acceptable levels.
You can also go with a tip one size larger than the one you presently have. This will increase the particle size and reduce the fugitive particles landing outside your target areas.
Let's not get too caught up in technical details; it sounds like you know how to spray.
Using the retarder properly is a big key, and Kremlin is right with suggesting a one-size-larger tip. I always went a size bigger, just for the added volume and speed.
Two other solvents that are good to use as a retarder are PM acetate or IBIB.
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