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What should I use to thin Rustoleum when I shoot it through a gun?



You use automotive enamel reducer, say medium speed, this is for temps of 55 to about 80 or so degrees farenheit.
You can also add an acrylic enamel hardener, such as Evercoats DOI Wetlook Hardener.
The mix ratio is 8 parts Rustoleum, 4 parts reducer and 1 part hardener.
This will give you a super glossy finish that will be very durable with the hardener added to the mix.....