Spraying Paint: The Details

Tom Patterson responded to my previous post with a question about the details of how get acrylic paints to spray properly.  I decided that I should write up my response as a post.  So here goes.

Tom, I feel your pain.  Getting acrylic paint to spray properly can be a nuisance.  I have to come clean: I don’t have “formula” because, truth be told, I’m an improvisor/experimenter with everything I do (some would say “hack” 🙂 ). Keep that in mind when you read this post, think of it more as a possible starting point for a conversation about spraying acrylics, rather than the advice of some kind of expert.

I have a theory about spraying acrylic paint.  I don’t think there is any kind of formula that will work every time.  With enamel paints, you’re spraying things that don’t occur naturally in the atmosphere.  With acrylics, the paint medium is made up of water, which is suspended in the atmosphere that you’re trying to paint through.  Because of this fact, acrylics are more directly affected by barometric pressure, humidity, and temperature.  But I’m no scientist, so all of this is simply conjecture on my part.  Basically, I’m saying that acrylics need to be fine tuned every time I paint.

For the Penn Central covered hopper in yesterday’s post, I used Polly Scale Penn Central Green, thinned down with tap water.  People have shrieked in horror when I tell them I’m thinning acrylic paint with tap water, but maybe my water has a mineral/pH level that’s conducive to thinning acrylics.  If your tap water isn’t working for you, try distilled water, because everyone says you should use that instead.

I apologize in advance for sounding pedantic; I’m going to make no assumptions and simply describe my whole process, rather than skip over things that I think you already know.

To thin the paint, I start by mixing the paint in its jar with a disposable stir stick until I’ve eliminated all, or nearly all of the lumpy stuff at the bottom.  Then I dump the paint into one of a couple of small mixing jars that I keep next to my paint booth. At this point, I work toward getting the right size of atomized paint coming out of the airbrush.

In the case of Polly Scale, I generally do a 2:1 ratio of paint to water right away, because it just simply looks too thick to spray when it comes out of the bottle.  I guess that part is just experience.  However, I know that the deeper I get into a bottle of Polly Scale, the more water I have to add.  That’s another reason why I don’t have a formula… the paint doesn’t keep the same consistency as I work through a bottle.

So I stir that basic 2:1 mixture in the mixing jar.  Sometimes add a drop or two of retarder.  I forgot to do that with this particular project.  Lately, I’ve been mixing gloss into the paint colour before I start thinning the paint down.  Again, I neglected to do that this time.  It was just paint and water this time.

Once my thinned paint/retarder/gloss/water is all mixed up, I take a few drops and shoot it through the airbrush.  If I don’t like how it’s spraying, I dump out the bit of paint that’s in the airbrush, spray some water through it to purge it clean, then I keep thinning the paint and trying it out until I get the atomization I like.  Basically, I start with moderately thinned paint and keep making it thinner until I like what comes out of the airbrush.  To keep this process running smoothly, I keep a beaker of water and some disposable droppers right next to my paint booth.

In the end, I really don’t know what the ratio of paint to water was yesterday because I just kept going until I liked the droplet size.  My guess is that it was nearly equal parts, or maybe something like 3:2 (paint:water).  Now, I recognize that this process sounds tedious.  I don’t always have to do it this way.  Through experience, I can get the right viscosity pretty quickly.  In the case of this covered hopper, it was about the viscosity of Vallejo paint straight from the bottle (by the way, buy a couple of bottles of Vallejo and practice spraying them – they’re better formulated for spraying).

I set the regulator really high (about 30psi).  This gives me very finely atomized particles of paint, especially with the Iwata.  I had to continuously apply very light coats.  The paint was drying within about five to ten seconds of landing on the model.  I had no problems with clogging or paint drying on the tip, but it was right on the verge of drying too quickly.  I think this is OK as long as the paint is wet when it hits the model.  I kept checking this by holding the model close to the light in my paint booth.

When I was finished with the colour coat, I shot a heavy coat of Testor’s Model Master Gloss, barely thinned.  Then I sprayed the trucks with Polly Scale Grimy Black, straight from the bottle (I took the trucks off and painted the car and trucks separately).  I wasn’t too concerned with getting the paint droplets really small on the trucks because I like a bit of coarseness here, so that there is a lot of “tooth” for the weathering.  With thicker paint, I find that you need more of a shotgun approach, which also produces more overspray and waste.

Again, I don’t want this process to sound overly laborious, but I get the sense you were looking for some details.  I wasn’t sure which details were more important to you, so I erred on the side of providing too much.

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6 thoughts on “Spraying Paint: The Details

  1. Hunter,

    Thanks for the detailed response- it’s much appreciated and very informative. I hadn’t considered the effects of the ambient atmosphere when spraying acrylics, but I can certainly see how it could have an impact. I’ve read about problems using tap water- glad you’re having success with it. I’ve also read about thinning acrylics with distilled water and even Windex. I have some bottles of Tru-Color, Vallejo Air and MicroLux Custom Blended paints that I’m going to experiment with over the next several weeks. With any luck, I’ll find a process that will work as easily and quickly, and produces the same fine finish, as the old Floquils.

    Tom Patterson

    • I remember Floquil being much simpler to use. Thin it down and paint. Done. I don’t miss the fumes. I like spraying Polly Scale, but we’ll have to look elsewhere now that Testors is done with that brand. I’ve been trying out everything I can find.

      • Hunter, I’ve used plain tap water now for years while painting out at the club facilities in Maryhill, and likewise with no issues. What I’ve had issues with were thinning with Windex, isopropyl alcohol and any pre-formulated acrylic thinners if mixing brands (ie Modelflex thinner with Polyscale paints). OK that last one makes sense, but the other two were supposed to be alternatives since tap water was apparently is a no-no. Now the water at the WRMRC is well water versus the regular municipally treated stuff, but still very good to hear your Hamilton-area tap water produces great results. I guess it all depends on the water quality of the municipality, with distilled being the safest bet.

        • Ted, I remember trying to thin Polly Scale paint with isopropyl alcohol and having the paint get like muck. I sometimes use Windex to clean the airbrush, followed by a generous rinsing with water. I vaguely recall someone telling me they use some kind of enamel thinner to thin water based paint. It seems odd to make a relatively safe paint much more dangerous by adding toxic thinner.

          The other thinner I’ve heard of is windshield washer fluid, which is glycol, I believe. I’ve not tried that because my results have been satisfactory without having to make the paint more toxic.

  2. Which retarder are you using? I have only limited airbrushing experience (weathering rolling stock and rails using Tamiya paints (thinned with alcohol)) and have never used any. I’ve just bought some Vallejo Air paints to try (I’ve heard good things about them — thin with water). Does your retarder work with both these brands/formulations?

    • Colin, the Model Flex Extender product works well. I’ve also used a Liquitex product that is much more economical. Regardless of the product you choose, the amount of extender that goes into an ounce of paint is so small that you’ll probably spill the bottle before you use it up.

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