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Im painting some parts and for some reason I  have these splotches or rings in the paint. I'm using tamiya x18 thinned 2:1 through my airbrush. The plastic was finished in 1000 grit and grey scuff pad. No primer.

Does anyone know why this happened?  Is it becase I didnt prime first ?

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It is most likely moisture in the air supply to the airbrush. Do you have proper water trap installed? If it is at the compressor  output you may need on closer to the airbrush, such as the Paasche inline trap.

 

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Yes, parts were washed. I  used the tamiya acrylic thinner.

 Moisture in the line may be the problem.

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Yes, parts were washed. I  used the tamiya acrylic thinner.

 Moisture in the line may be the problem.

Think of an aircompressor as being a bit of a heater, simply because compressing air (or any other gas) does heat up the air at least a little bit.  Now, think of that airbrush hose as a condenser, as it has air moving through it in which the temperature (at least to a little bit) will cool off as pressure is relieved.  In addition, the hose itself can be at least few degrees cooler than the compressor cylinder (and the storage tank as well).  This can allow moisture (from the humidity of the ambient air in your room or workshop (or outdoor air if you work in an area that is not otherwise climate controlled).

A moisture trap mounted directly at the air outlet of a compressor or air tank can't collect condensed water from the hose, as the hose is "downstream" from the moisture trap.  This is where an "in line" moisture trap does its best job--down the line from the tank or compressor, with a length of hose to allow water to condense out of the air that has had its pressure released.  The condensed water droplets can flow down the inside of the first section of hose and be collected in the water trap, allowing drier air to flow beyond, to your airbrush.

I got my first airbrush setup (Binks Wren, with diaphragm compressor and a length of hose, WAY back at Christmastime 1961.  My bedroom (and my model building area) were in the basement of an old house (I live in mid-north Indiana, where it can be humid 24/7, 365 days a year--plus one more for a leap year), and I soon discovered that I was getting tiny droplets of water spitting intermittently out of that Binks airbrush.  Fortunately, one of our neignbors was both a commercial artist (Mr. Nelson did advertising art for our local newspaper back then) and was a good customer on my paper route.  As it was he who suggested I was ready for an airbrush, I asked him what I was doing wrong.  His answer?  Get a water (moisture) trap and a second length of hose to connect the water trap to my compressor--so I spent another $16 (a fair amount of change for a 17yr old in 1961-62!) hooked it up as Walter advised, END OF PROBLEM!   

The Binks Wren airbrush died about 35 yrs ago, and that Binks compressor went to Air Compressor Heaven a few years later,  but I still use that solid brass Binks water trap to this very day--and no matter the humidity, no matter the season of the year (and I still live in the same city), I never get droplets of moisture in my paint.

 

Art

 

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