As much as I swear by good 'ol Testor's Dullcote, far as I'm concerned Alclad Klear Kote Flats are far superior. They airbrush like a dream out of the bottle, and the ALC-314 Flat is the flattest flat I've ever seen.
Though I make sure to keep all 4 in stock, I've always figured one could just have the 311 and 314 and probably mix any degree of flatness in between by mixing.
I have never once had this product line frost up due to operator error, I surely have with many other clear flats.