Hi, question.
I always put thinned acryl painting into my airbrushcup using a pipette. Quit often there's a lot of bubbles rising over the rim of my airbrushcup while spraying. What's going wrong????

sounds like some of the air is going back, maybe a blocked tip or an old rubber o-ring in the tip?

Try to dissasemble and clean up, check your airbrush sealing/orings. Do not use too aggressive specifics to clean up. Some of them may dissolve orings.

Could be. I must admit I have a very cheap set but it worked fine (untill now) airbrush

I had two cheap copies of Iwata. Paid around 35-40Euro for one and had problems with both. Bend or broken needle, broken nozzles, problems with orings always trouble. Then I bought HS Evolution 2 in 1 almost 4 years ago. Did more then 20 big models with bases (1:35 armor and 1:32 planes) and it still works like new. It cost me 140euro so it was worth every eurocent I paid. Consider buying a better airbrush. I'm sure you will quickly notice a difference.

The air is backing into the paint cup from a blockage. Try taking your airbrush completely apart and soak the parts in alcohol for at least a day. buy a set of airbrush cleaning brushes, you can get it from a art supply store. clean everything till there is no residue of paint. What kind of airbrush are you using?

Revell Starters Kit, well, not so cheap ( 135 incl. compressor ). Let's start by cleaning it very well.

I've had the same problem recently.

The issue was: the small spray nozzle was broken. I've been cleaning the nozzle with to much pressure, so the tip reiceievd a mico crack. It's hardly visible with eyes, but enough for air to flow back to the bottle.

So, probably you'll need a new nozzle

After using many different types of airbrushes for over 50 years now, and working in a LHS where we sold airbrushes of various types, I've always stuck to the philosophy of buying quality airbrushes. So often modelers buy cheap brushes and then have all kinds of problems. It's better to start with a more expensive higher quality airbrush and compressor than to find you've wasted money on a cheap brush, or worse, trying to obtain parts for a no-name brush. I've seen many first time modelers turn away from the hobby because they couldn't use a cheap airbrush-- had they invested in a higher quality brush, they probably wouldn't have been so discouraged. I agree, it sounds like a tip problem, it could be a micro-crack as mentioned above (you may need a magnifying glass or jewelers loop to find it) or it may be a cross threaded or unseated tip, or as someone else mentioned, a seal at the front of the brush. If the bubbles for when there is no pressure on the trigger, it could be a problem at the point where the air enters the brush. Or it could be a blocked tip. I recommend cleaning the brush after spraying every color-- especially true for acrylics-- by flushing with either the manufacturers thinner or lacquer thinner (for enamels). Then after each airbrush session, breaching the brush down into its smallest components and giving it a thourogh cleaning.
VR, Russ

If it starts after you've been painting for a few minutes, rather than starting as soon as you begin painting, then the problem is that paint is drying on the needle and clogging the nozzle. This is pretty common with acrylics. The simplest solution is to carefully wipe off the tip of the needle and you should be back in operation until it clogs up again (repeat). You can also try adding some retarder medium or airbrush flow improver to your paint to prevent this or at least slow it down. And of course what everyone else said--clean the airbrush thoroughly after each use.

Thanks for all the tips. I took it apart, soaked it in Acryl thinner, bought a new rubber ring for 0,25 cents and a set of Interdental brushes (2,5 mm) for 0,99 cents to clean thev openings (and a lot of rubbish came out) and it works like never before !