I always reduced One Shot with a high quality paint thinner or sometimes turpentine. However, when pinstripping, all of us strippers would add a bit of lacquer thinner. Automotive acrylic thinner also. Well, some won't admit it. Some guys just don't want to give up information.
The lacquer thinner would allow for a better bite with the brush and no sags, especially where you start your line.
For pictorial, lettering, or outlining, best to have the correct mix, enough to get the image completed, or at least the first coat, if a couple of coats is needed.
Most sign guys, myself included, would work with two tiny cups and a cardboard palette, generally held in the left hand. Between the cup with the paint, and the cup with the turps, the consistency could be controlled.
Some colors you would have double coats, like white on a black truck. There are tricks. We would add a bit of silver to white. With a fully charged brush, and a bit of talent, you could stroke a letter and cover in one pass. Get a lot of lettering done quickly that way.
The best thing was, "Don's brushing lacquers." Totally lacquer based and brushable. Could be sprayed also. Covered really well. Just got discontinued.
Lot of sign guys went PC early, especially the late bloomers, who didn't have the long background with the hand lettering craft.
My guess is, the chemical in the One Shot cans available today, isn't quite like what I used for years, I'm told the lead has been removed from the formula. There's water base One Shot also. Unless they stopped making that?
One thing I had no difficulty with, was calling for tech help. Products and systems would change so quickly, you were forced to seek correct information.
CB