Bottle w/brush touch up paint question?

After awhile, the paint tends to get thick. What can be used to thin it out? I've tried laquer thinner with no success.
BTW, I have a Harbor Freight inexpensive air chisel. I put Velcro in the groove
of a chisel and the mating piece on the side of the touch up paint bottle. Press them together, WRAP THE BOTTLE/CHISEL WITH A TOWEL AND HOLD IT. Turn on the air chisel. I usually let it run until the compressor turns on. Actually, the paint is probably shaken more in ten seconds this way than a half hour by hand.
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