There are usually two types of walls. The walls in older homes were often constructed of plaster and lath strips. As the house settles or nails are driven into the walls to hang pictures the plaster can crack. Most of the time the cracks are hairline fissures which are easily repaired. Cracks in plasterboard can also be caused a house settling, by driving in nails for picture hanging if the nails are not put in where the wall studs are. Cracks can also be located where the individual plasterboard sheets are hung. Changes in moisture can also cause plaster to crack.
Repairing cracks in plaster walls
If you have plaster walls with wide cracks, use a utility knife to clean the debris (crumbled or broken bits of plaster) out of the crack.
If it seems like the wall around the crack is warped, reattach the lathe by nailing nails through both the plaster and lathe. Then the cracks can be filled.
If you bought the plaster paste as a powder, mix according to the directions on the package. Using a putty knife, fill the cracks with a small amount of paste at a time. Sometimes, if the crack is very tiny you can apply the paste with a butter knife.
Let the plaster dry for several hours and preferably overnight. Take a piece of fine grained sandpaper and sand the area to remove any excess plaster until the wall is smooth. Then you can paint or paper the wall.
You will need to use mesh tape or joint tape for a very wide crack for the plaster paste to stick properly and effectively repair the crack.
Cut enough tape to cover the crack and extend about 5 cm (2 inches) in each direction. Use more putty and a wider putty knife to push the paste deep into the crack using diagonal strokes that cross perpendicular to the crack.
For a wider crack you definitely want to wait until the following day to sand and paint.
Repairing cracks in plasterboard walls
Cut out a trench slightly larger than the crack and then brush it with a paintbrush to get out the chalky dust and dirt where the plasterboard has cracked or been otherwise damaged.
Fill the hole or crack in the plasterboard with specialised spackling putty or joint compound.
Apply the compound in two stages as plasterboard can withstand more sanding. First, fill the crack or hole with the putty or joint compound. For small cracks and holes you can probably immediately sand the surface with a fine grained sandpaper.
A second application of the putty will make the surface identical to the rest of the wall.
Just as with wider cracks in plaster walls, a very wide crack will need a little more structure to stick properly. This is where patching tape, also called mesh tape or joint tape, comes in handy. Cut enough to cover the crack and extend about 5 cm (2 inches) in each direction. Now use more putty and a wider putty knife to push the paste deep into the crack using diagonal strokes that cross perpendicular to the crack.
After you have sanded the patched area smooth you can paint or wallpaper.
If you wrap the sandpaper around a piece of scrap wood, you can make a flat surface and avoid forming trenches or dips on the wall.