The paintbrush is a powerful magical artifact that can change obsidian into sand with a flick of the wrist. Only the mighty VoxelSniper is a more powerful alchemical tool!
To use the paintbrush, it must first be initialized with a left-click. Once it's initialized you will get a Pallet Ready! message. Now you can pallette most any block type with your next left-click. When properly palleted, your brush will say Ink acquired! (Stone Double Step), or whatever blocktype you palleted. Now that your paintbrush has been palleted, right-click a new block to paint the palleted blocktype onto it.
Note that paintbrushes store their inks based on what inventory slot (1-9) they are held in. Like the Dooplicator, the paintbrush does not work on TNT, water, lava or other restricted material types. A video tutorial is included on the VoxelDoop article.
VoxelDoop Tutorial Video