I have two objects each with it's own material. I joined the two objects, that gave me one object with two materials. However when I erase the materials and try to put them back on only one material applies. how do i fix this?
I suppose the problem is the following: you have a mesh with two materials and you want to change these materials but keep respectively the faces that correspond to the first and to the second.
Say we have a mesh with two materials RedOne and BlueOne, like this:
If you remove the material slots and add new material slots, you will lose which face is correspond to which material:
Instead of removing the slots, you should assign them directly the new materials, because faces of the mesh are assigned to slots (and so you keep face/slot correspondences).