I use SG and if I am having sputtering trouble, I strip it down and soak the components overnight. That usually does the trick. Be sure to backflush the airbrush when done and then run plenty of cleaner through it.
Also -- sputtering from poor paint is NOT helped by thinning with water. If the paint is old use Createx transparent airbrush extender. It usually cures the sputtering and allows nice fine spray. DF if you are running 40 lbs. your paint is too thick. That will also cause uneven coverage and blowouts.
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