I have a 1.5" pvc rough in the wall at the site of my wife's proposed new laundry sink. The stub is only about 3/4 or 1" long and it is capped with a plastic cap called "Cap On (pat. pending)" around the flange is says to "knock out with hammer". What happens when I knock this out? Are there threads under the cap or will I need to cement an adapter to the stub with threads to match the compression fitting for the p trap? Does the cap come off altogether? Naturally the whole thing has been sprayed with drywall texture and at least two coats of paint!! Thanks for any light you can shed on this.