Your problem is that you have ngons that create this kind of artefacts. Also you have a lot of useless edges, if you use a Subdivision Surfaces modifier you won't need so many.
If I were you I would redo the object, it would take less time than correct your version:
- Keep the profile only, mirror it, use the Checker Deselect and the Dissolve Vertices tool to keep only the necessary amount of vertices.
- Now extrude the profile, give it some edge loops and bevel where the cut are supposed to happen, then delete the faces.
- Give it some additional edge loops to make the holes square, give your object a Solidify modifier to give it thickness, keep a copy of the object somewhere, apply the Solidify modifier, bring some additional edge loops to sharp the edges again.