A corner finisher spreads mastic on a joint formed at an inside corner of drywall. The drywall corner finisher is conventionally used with a plurality of alternative tools for supplying a plurality of coats of mastic in different thicknesses. Therefore, a quick disconnect coupler is provided to rapidly change the tools and yet to hold them so securely that there is almost no chance for an unwanted disconnect between the drywall corner finisher and one of the alternative tools for supplying mastic. The invention provides such a coupler in the form of a ball and socket behind a sliding locking plate having a key hole slot in it for receiving a ball on a mastic delivery device.