How to Attach Trim to Old Plaster Walls

Use an electric metal scanner to find the studs behind the plaster walls. If the plaster wall was attached to the studs with nails, you could use a metal scanner to detect these nails. To do this, hold the scanner against the wall and slide it across the wall surface until it beeps, which indicates that it found the edge of a stud. Slide the scanner to the other side to find the other stud edge and mark the center of the stud.

Try hammering nails into the walls to find the studs if the metal scanner doesn't work for you. In some cases, the builders did not use nails to attach the walls to the studs, so a metal scanner can't detect the studs. A stud is 1 1/2 inch thick, so try driving nails into the wall every 2 inches until you hit a thick, solid wooden surface. Once you find one stud, the other studs should be easier to find because studs are usually installed about 16 inches apart. Do this where you will attach the trim so it could hide these nail holes after installation.

Hold the trim against the plaster wall, then drive a finish nail through the trim, through the plaster wall and into the stud. You could use a hammer to do this. Pound the nail into the trim until the nail head is about 1/2 inch from the trim. Then, hold a nail set against the nail head and hammer the nail set until the nail head is about 1/8 inch below the trim surface. Alternatively, you could use a power nailer to save time. Continue driving one nail into every stud you find.

Fill the holes over the nails with wood filler to help them blend in. Once the wood filler dries, sand the filled areas to smooth the surface of the trim.