How do you keep thinned paints

In a closed bottle. You can get Testors mixing bottles. Also, it's not recommended to put thinned paint back in the original bottle with unmixed paint long term, long term storage can ruin the drying agents that way. Or to keep mixed paint long term. I've done this for several months at a time though but that really is stretching the limit.  Don't plan on long term storage of thinned paints. In a sealed jar you should be fine through the duration of a single build.

Back around March or so I mixed up a batch of MM enamel thinking I was going to spray it within a month. I finally shot half of it today lol ( 6 months later) ! Hopefully get to the rest by the end of the week. It went down well, leveled out, seems to be drying fine. That's the longest I've ever done that for and I have to say I was a little concerned it might have turned to crap.

Or you can try mixing up smaller batches of paint for each airbrush session. Once you thin paint, storing it has a definite shelf life. As long as you’re not mixing a custom color, mixing up smaller batches to reduce wastage is not a bad idea. 

One other thing I would do with excess thinned paint, rather than store it, was to try out a technique on a paint mule. It didn’t matter so much what the color was, but rather the angles, distances, hand movements and settings of the airbrush. 

Usually I mix enough paint with intention to have a bit more than I will actually need, especially when I have mixed to match a color. I plan to dispose of the excess, when I'm certain the job is finished. Storage of paint has never really worked well for me.

That said, I have had the most success by making sure that the storage container is completely clean, with the threads of both the cap and jar having no residue. I clean and save some of my emptied glass jars, just for short term storage of mixed paint while a project is progressing.

If air is allowed to enter a storage jar that contains paint, thinner and other common ingredients like retarders/accelerators, it won't take more than a rather short time for it to become useless.

Patrick

I use a Badger suction feed brush.  So when I start a kit I figure out how many colors I will be airbrushing. I then pull out that many airbrush bottles and mix up/thin the colors. After finishing the kit I clean out bottles and return to my supply.

I find I can only clean a bottle so many times before I find them too hard to clean, so have to buy a new package of bottles now and then (package is dozen bottles).  I find it is not a major expense.  I also find I cannot keep thinned paint in those bottles for too long a time before it goes bad, but only time that is a problem is when I leave off building a particular kit for a long time and try to go back to it.

It may sound counterintuitive but thinning a bottle of paint, particularly organic solvent paint, will often lead to premature solidifying in the bottle.  

Out of the bottle I will only thin what I will be using in the near future.   It is stored in a separate small bottle - I have a lot of Blue Apron liquid bottles. 

With custom mixes I also mix in the small bottle what I think I'll use.   I spray out a bit as reference for later mix standard

Paint is relatively cheap.   Dont try to stretch it by thinning the whole bottle It will be hard and unusable when you really need it    Clean the threads on the jar and keep it tightly sealed