Drywall – How to fill in a missing strip of drywall that I uncovered after removing a piece of trim

You want to add backing to the inside sides of the existing studs as well as to the centre of the span. The simplest way to do this is to cut the new backing (preferably 2x4, or whatever the existing stud dimensions are) about 4" longer than the height of the opening and screw them to the existing studs. The cut one more piece of backing to the same length as the other two, but this time place it in the centre of the opening and screw through the top and bottom of the existing drywall to attach it. See the picture below for reference.

This method will be much easier than trying to score the existing drywall back on the existing studs and will provide a solid patch.

Paint that has not properly adhered to the surface below could definitely peel off with the slightest touch. Depending on conditions, any masking tape might cause this; however I would suggest using high quality painter's masking tape to minimize the chances.

The soft material you have encountered under the peeling paint is the drywall joint compound used to cover edges, corners, and joints when drywall is installed.

I am afraid that you are in a tough spot now that it has begun to peel. You need to make sure the paint that is most unstable is removed, or it is likely to peel off in the future, including when you repaint the wall. Then you will need to:

• sand the areas where sound paint meets substrate until the interface is smooth, then prime and repaint.

OR

• use a wide trowel/mud knife to apply one or more coats of drywall finishing mud (sometimes called topping compound) to the peeled area, sanding after each coat, until the wall is smooth and free of visible imperfections. Then prime and repaint.