T-4 is in the catalog because of customer demand. As Maynard Buehler said: THIS is the BEST yet. Every time a new gun blue comes on the market, we test it on a piece of gun barrel. We blue three spots: one with the new blue, one with Oxpho- Blue and one with T-4. One day later we start rubbing all three areas with steel wool until one of them starts wearing off. So far, Oxpho Blue is toughest and T-4 next. However, for ease of application and deep density of color, T-4 is superior. Its ability to touch-up wear areas and scratches instantly has made it a favorite.