Ever have a crack in a wall or ceiling patched and the next thing you know it’s back? The surface is moving and forcing the patching material to bend or stretch pass what it is capable of. Most of the time it isn’t practical to tare into the wall and if you did, it may not solve the problem anyway.
Many patching materials on the market say they stretch or are flexible. So why, when you use them to patch your crack, do they crack back out? Obviously, the crack is moving more that the material will absorb. We usually smear a thin layer of the patching material into the crack and it fails.
If a material says that is will stretch 200%, it has to be at least as thick as it needs to stretch. Also, what you are patching needs to be strong enough to hold onto the patching material and not break off under the tension. You see this at door frames sometimes. The caulking has pulled off one side or the other and still has some of the other side stuck to it.
So, you need a certain thickness and a good grip. Let’s use a patch in a drywall wall or ceiling as our crack repair.
First, drywall isn’t strong, so to get a better grip we are going to apply a layer of fiberglass mesh drywall tape over the crack. This will give our patch a bigger area to grab.
Second, apply a layer of “Knife Grade” elastomeric patch. Knife grade means that it is thick like spackling paste. Apply a thin coat smoothly over the tape. Just enough to cover the tape and get in all the pores of the mesh.
Use a flexible plastic spatula like that used for Bondo. The material doesn’t sand well because it is rubbery and a plastic spatulas will conform to the wall better. A soft brush, dipped in a little water, will help smooth out the material before it drays. After it dries you’ll need to cut it with a blade.
Third, apply a second, wider coat after the first one has dried. Smooth while wet as needed.
If the tape is covered, you’re done with the patch and it will be flexible. However, it may not match the texture of the wall.
Not all textures can be simulated with the elastomeric. You may have to use regular wall texture. The wall texture will stick just fine.
It will, however, crack if it is forced to flex. So will the paint. The cool thing is that when the wall flexes the gap in the texture or paint is white and not a black crack, easily touched up if you desire.