Dynamics Joint Putty Knife

A 6 inch joint knife is the tool for joint and crack work ahead of a coating. It presses semi-rigid filler and patch material into control joints and cracks, works out the air pockets, and pulls the surplus flush with the slab so nothing sits proud where a roller or a squeegee has to pass over it later.

The width is the point. Six inches spans a joint and bears on solid concrete on both sides, so you can shave filler flush in one pass instead of following the dip and leaving a low line that will telegraph straight through the finished floor. On a flake floor a proud or sunken joint fill is one of the few defects the chip will not hide, because it changes the plane rather than the color.

Timing matters more than technique. Most joint fillers are shaved once they have set enough to hold shape but before they harden fully, and that window is short. Have the knife ready before you place the filler, keep it clean, and wipe it down before material cures on the blade, since a nicked or resin-caked edge stops pulling a clean line.