I stored a thinned Tamiya acrylic paint in a Paasche siphon-feed airbrush jar after a painting session 2 weeks ago. I covered the connector with a vinyl cap and the lid hole with a tape to prevent air from getting into the jar.

Today I opened the jar, stirred the paint for one minute, and began AB'ing. The spray was extremely grainy. I thinned the paint more and stirred more. The result was the same. Does this indicate that air somehow got into the jar?

No not air it's just that the paint has gone bad. Throw it out and start with a fresh batch and mix only enough for your current needs.

Howdy..

Not sure if this helps figure out what went wrong or not, but I just checked my siphon lids and none of them had the insert seal that regular paint bottles use to get an airtight seal, so I would think that maybe your paint was slowly drying out and went bad.

Delbert

What type of paint is it? Either way you have to replace the lid with a better sealing cap. But you should not store mixed paint for any length of time.

To add shelf life to Tamiya acryls mix in a few drops of glass cleaner, at the beginning. Once it has gone bad, there is no recovering it. On a side note, I used to store paint in the siphon bottles with a regular cap. It kept for a few days, without glass cleaner.