What Am I Doing Wrong

Sounds like the paint is drying out to fast at the tip. Keep a Qtip close at hand with thinner or Windex on it to swab the tip (pull back the needle, don't use air!). You can also use an acrylic retarder that slows down the dry time. Model Master Acryl paints are supposedly pre-thinned, try not thinning at all. Thinner speeds up drying.

http://www.craigcentral.com/models/ is full of great advice on thinning, airbrushing and acrylic paints.

Bgrigg is right, get some acrylic retarder. Also, if you need to spray a fine line, you need to really thin that paint down and lower the pressure way down. If I read your post correctly, you said you use a 4:1 Paint:Thinner ratio @ 15-20 psi...You need to thin it a lot more. I go anywhere between 1:3 and 1:7 Paint:Thinner ratio @ 10-5 psi and I spray 1/4 to 1/8 of an inch to the model...relatively close. That's my on getting a fine line.

I'd definitely play with the paint to thinner ratio, as 4:1 is way too high. I usually mix 50:50 with Gunze.

You might want to try Tamiya Acrylic Thinner, as it contains a retarder which prevents the paint drying too quickly.

Karl

i don't know how fine a line you need but you might just try using a medium-sized nozzle.  but 4:1 thinner definitely sounds kinda thick.

I'm at the opposite end of the spectrum...

I tend to run thin paints at high pressure and low volume.

I'll spray acrylics at 30-40psi, thinned so the paint has the consistency of milk. Anything more and it splatters and clogs the tip, anything less and it spiders.

If your paint is drying too quickly, using a retarder is one way to fix it, but...when I thin paint, I  typically use a 50-50 mix of water and rubbing alcohol as my thinner. Try increasing the amount of water in the mix.

Jeff

Jeff Herne wrote:

I'm at the opposite end of the spectrum... I tend to run thin paints at high pressure and low volume. I'll spray acrylics at 30-40psi, thinned so the paint has the consistency of milk. Anything more and it splatters and clogs the tip, anything less and it spiders. If your paint is drying too quickly, using a retarder is one way to fix it, but...when I thin paint, I  typically use a 50-50 mix of water and rubbing alcohol as my thinner. Try increasing the amount of water in the mix. Jeff

Do you have to move fast? @ 40 PSI, you are shooting a lot of paint.

Not really, I said high pressure and low volume...I can do extremely fine lines with my Iwata. By minimizing the amount of paint,  the higher pressure provides much more control and minimal overspray. I've gotten to the point where I can spray a complex scheme without masking using this method. Obviously, for sharp lines, I need to mask, but soft-edged camo patterns are no problem.

Jeff

Does dish soap really work for a retarder?

That's what I've used, but I'm only using a single action, external mix airbrush right now. Are there other acrylic retarders?

Regards,

yeah there's several paint mfgr's out there with retarders.  Createx, Liquitex and a few others I can't remember.  I have the Liquitex Airbrush Medium and it works well.  I got it at Michaels...

Something else that works good for tip dry is airbrush needle lube, Badger Regdab Needle Juice or Iwata Medea lube work great and are only about $5 for a jar that lasts forever!  It helps give a surface the paint has a harder time to stick to.  Just a tiny thin coat of it over the front 3rd of the needle does it.  You can also put a drop or so down in the trigger assembly to give a smoother pull on the trigger.