Enamel Clean Up

Let me see if I can help.  You don't need a lot of thinner.  I keep a special bottle of cleaning thinner (I use lacquer thinner) and re-use it.  Pour a little in a glass cup or bowl.  Wipe out the airbrush cup with a paper towel.  Put a dropper full of thinner in the airbrush cup and swish it around with an old paint brush.  Shoot it out.  I use an empty plastic jug with a small hole in the side and a rag or filter over the top to catch the mist. Repeat a couple of times until the thinner comes out clear. Then clean the cup in the bowl of thinner. When you are done, pour the excess thinner back in the bottle.  Most of the pigment will settle to the bottom by the next time you use it.  I don't have a Paasche H, so I might be missing something, but I hope this helps.

I always wear an organic vapor mask and nitrile gloves when airbrushing. 

Don 

I do basically what Don said in post.

Other than what evaporates or is wiped off on paper towels, I never dispose of used thinner.  I recycle it using two 1 quart jars.  In one jar, I store my thinner that I use for cleaning, and in the other jar, I store my dirty thinner.  After a day or two, the pigments of the paint in the dirty thinner jar will precipitate out and form a gooey layer on the bottom of the jar leaving clear thinner above.  Once this jar has enough clear thinner in it, I pour off the clear thinner into the cleaning thinner jar.  After a while the clear thinner takes on a greenish hue, but its cleaning effectiveness is not degraded.

When cleaning out my airbrush, I use an Aztek cleaning station (no longer use the Aztek AB, but the cleaning station is good!) which allows my to spray thinner through the airbrush and collect it rather than simply spraying it into the atmosphere.  You can either make one as Don described, or you can buy one such as this Iwata Cleaning Station.

Your concern about waste is valid, and for me, during the past ten years of building models, I started with about half a quart of thinner from a previously opened can that I had on hand, and during the following years, I've only bought a single quart can of thinner.  Using ty recycling system, I still have well over a quart of thinner between the unused stuff till in the can and the used stuff in my two jars.  Added up, that means I've actually used only about a pint of thinner during that time.

Andy