As stated, Xylene is a good reducer for industrial enamel, (Rustoleum).
Just about anything will work to thin industrial enamel. But, you can lose gloss, if you use a lot of reducer, and or, pile on the coats.
Industrial enamel is the cheapest paint you can buy, they use very low quality solvents. It takes a long time for those solvents to evaporate. Paint doesn't "cure", (reach full hardness), until all the solvents are out of it. Putting your stuff out in the sun will help greatly, as the heat will help drive out those solvents.
Adding the correct amount of hardener, can help speed up the process. Keep in mind, more is not better. Adding too much hardener, will cause problems.
I suggest thinning as little as necessary to get the paint to spray nicely, with your equipment. This is almost always a good rule of thumb with any paint.
Secondly, allow each coat to "flash", (dry until it starts to get tacky), so as not to trap excessive of solvents in the film. Piling on the paint, will usually cause solvents to get trapped, loss of gloss, and slow down curing.
Apply enough paint, but more than two or three good coats, can cause trapped solvents. Excessive film build is also not better.
Use the proper protective equipment. Including, at minimum a charcoal filter respirator, when spraying industrial enamel. Disposable respirators only sell for around $25. And, if kept in an air tight container, will last about 40 hours of use.
