Since you are in AZ, you need to be sure that the heat will not effect the paint or wall paper. We have put some paper up in one a while back and you need to be sure it is well struck. Clean the existing walls really well, then apply a good coat of Shielz. If you plan to paint, put on the Shielz first.
What you have to be concerned about in either case is that the coating that is there now is going to stay stuck. Since it has done OK this far, I would not expect much proplem in that reguard.
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