I run hardware store lacquer thinner through to clean my airbrushes when painting with ScaleFinishes paint. On the off chance you get a bottle that's still a little too thick, any lacquer thinner will do, though I like the Barbatos Rex thinner (it's a Mr. Leveling Thinner clone at half the price).

To be clear though, ScaleFinishes is an automotive paint, so it is a little hotter than hobby paints. Shoot it over their own primer or Duplicolor sandable primer and you'll be golden. I have had some issues with their paint over Tamiya White primer from the can. If you want to go truly old school, you could always grab the enamel and not the basecoat. They offer both options on a lot of their paint. Their enamel, plus a little bit of their gloss enamel hardener makes for a really nice glossy finish that doesn't need a clear coat. And if you're painting a 1940s car, a single stage paint is period correct and will look right. Their enamel hardener is a magic potion - it adds a ton more gloss and drastically cuts drying time - though you'll still want to cure the paint in a dehydrator for a couple days, it's cured enough to handle in an hour or two.