I've recently picked up modelling again, after quite some time. Dusted off the old airbrush, cleaned it inside and out.

Now I've started with laying down some primer but I get a sketchy pattern, see the pics below:

Can anyone give me a tip about what I'm doing wrong? (PSI was 15-20 and it was airbrushable primer).

http://i68.tinypic.com/29gfvow.jpg

http://i66.tinypic.com/2cyfe2x.jpg

Some questions I have to get a better understanding:
1. Did you wash/clean the model before priming?
2. What type of paint are you using? Acrylic or enamel?
3. Did you thin the paint?
4. About how far away are you when spraying the surface with the airbrush?

Could be too wet, too thin, too high pressure.

Possibly it looks like you've laid it down to wet and thin, then subsequent passes have blown the still wet paint around on the surface.

More information would be helpful though, you haven't given much to go on.

As a model paint manufacturer it is hard to tell what the problem is. First, are you using any thinner; if yes are you using the same company's brand to thin ? That is very important as thinners are not compatible from manufacturer to manufacturer, usually. If it is a solvent based paint you may be spraying at too low a pressure (we recommend 28-35 PSI for our paint to get a smooth, even coat) causing the paint to dry too fast.

Then there is cleanliness of the part as the other commentators said. Needs to be clean to get good, non-blotchy appearance.

To be more specific as to the cause we need more info. What is the part - cast or molded, paint type, thinner used, distance from model, tip size on the air brush.

Trust this helps.

Martin Cohen, PhD
Tru-Color Paint

That's my thoughts too. Had the same results once, too much thinner and too much opening on the needle.
Use more air to when spraying. Adjust the needle forward (on a single action) or don't pull the needle back so much (on a double action). Too much thinner can usually be compensated by spraying with more air in the air/paint mixture.
Some paints require more thinner to be sprayable and then the use of more air (leaner mixture) can compensate for the "runny" paint.
Spraying at a slightly longer distance could also help, spreads the paint over a larger surface so that you get less moisture per unit of surface (square inch, square centimetre, thumbprint or whatever), in theory it also lets some of the thinner evaporate but that depends on what kind of thinner you are using.
/ Robin

Was it a fresh bottle of paint?If not could it have already have been thinned for airbrushing. What was your ratio of thinner to paint? What distance was the tip of the airbrush from the part?

From my vast experience in messing up, I'd say surface prep is most likely, the plastic is too shiny or there is some mold release/oil on it. Option 2 is that you used water as a thinner and nothing to kill surface tension. Then it tends to bead up like this. Try to do as someone said above, lots of air, little paint, effectively a dust coat. You are not looking for coverage, just adhesion. Then dry with air only, then the next coat can be wetter.

Maybe the dilution was right, but you put too much paint at once. As said, try to build up very thin layers and wait until they are dry before applying the next.

It could be any of the problems others mention in the posts above, but it could also be a needle or needle housing problem too. The patterns you describe and the "spitting" issues you seem to have can be caused when the needle is bent or the needle housing is split, which allows an uneven flow of air through the tip, causing higher and lower pressures around the air vortices at the needle end. It would help to know what airbrush you have. Paasche and Badger air brushes occasionally have a tendency to get hairline cracks at the tip in the needle housing. This is especially true as the airbrush ages. If you hadn't used your airbrush in a while, and it had particles of dried paint or corrosion in the needle housing, the needle might damage the tip as it's forced into the tip, causing a split. Grab a magnifying glass and inspect the tip closely.
VR, Russ