Self-adhesive fibreglass or paper tape is bedded into plasterboard joints, using three layers of compound, to help strengthen them and make them seamless.
Use a base coat compound for the first two layers, then a finishing compound for the last layer.
Or you can use an all-purpose compound for all three coats.
Fibreglass tape is the easiest to use, but it isn’t as strong as paper tape and is not suitable to be used with pre-mixed compound.
Use a 100mm broad knife to fill the recessed joint with compound. Roll out the tape over the joint, then gently press along the tape using the broad knife to bed it in. Allow to dry.
Apply a second coat using a 150mm broad knife. Allow to dry. Apply a third coat with a 200mm knife. Allow to dry, then sand flush with the plasterboard surface using a sanding float.