Mix equal parts of water and acid. When it stops fizzing, scrape or scrub with a wire brush and rinse. Repeat as needed. Be extremely careful with acid, and wear skin and eye protection. Q. I have to make a big patch in a plasterboard wall.
Trouble is, the plasterboard patching material is thinner than the old wall.
If the opening in the plasterboard shows studs, which you have to nail or screw the patch on, put shims on the studs so the patching material is even with the original plasterboard. Nothing new seems to match whatever is in or on the house. Apply a thin skimcoat of compound on this, smooth it out and let dry. Apply two more thin skimcoats, letting each one dry. Sand smooth and repaint.