Hey so i rigged this robot here and weight painted its arm. The joints are right i think because the other arm moves fine, but when i rotate this one the mesh becomes deformed. I don't know what the problem is, i've made sure to paint the enire arm too.
If you want a vertex to solidly move with a bone, assigning a weight of 1.0 is not enough: you need also to delete any weight referred to every other bone: It looks like the deforming vertices of your mesh are weighted also to some others bones. To check one vertex and see which weights it has, select it in edit mode and search in the "N" properties panel the "vertex weight" panel.