I guess i should shouldn't use the old spackle and tape. what should i use for cement board?
Mesh fiberglass tape with thinset mortar. Put the tape over the joint and trowel on the thinset. Make sure you don't work the thinset too much or it will cause the tape to move around and bunch up. The tape doesn't stick to the backer board very well in my experience.
Use mesh tape and thinset mortar. They make a special tape for this that is wider than the mesh you use for drywall. Hope this helps.
You can use the same fiberglass tape that is used for drywall, but you need to use thinset instead of spackle. It's done pretty much the same way as drywall, except you don't have to make it perfect since it will be covered, just flat enough that it won't interfere with tile.
