DIY Three Coat Plaster Repair
Also see:Three coat plaster consists of three separate components to create a hard, smooth finished wall or ceiling: the scratch coat, the brown coat and the putty or white coat. To restore this wall system all degraded wall material is removed until only sound plaster surrounds the damaged area. Then each coat that is ruined is replaced, one at a time.
It should be noted that this is a restoration procedure. Three coat plaster can be repaired more easily using one of the other wall repair techniques on this website. See the links to the right for skim coating techniques, plastering materials and wall repair procedures for plaster and drywall holes and cracks.
Lath Plaster Construction
The foundation for three coat plaster is the lath. Lath may be wood, metal mesh, masonry or gypsum board. The first wet coat is the scratch coat. This is a mixture of sand, portland cement and some filler material such as horsehair. This coat is applied with a very rough finish to accept the next coat, the brown coat. The scratch and brown coats are essentially the same, the difference being in the finished surface. The brown coat will be smoothed out more to accept the final plaster coat, the white or putty coat. Old white coat consists of plaster of paris and lime mixed together to create a hard, smooth finish.
We use wood lath for this demonstration but the plastering process is the same for all types of lath material. Repair the lath first if necessary, nailing 1½ inch wood strips such as lattice molding, over the existing wood framing to replace any broken or rotted lath strips. Leave a gap of about a quarter inch between each strip. As an alternative, drywall may also be used to replace wood lath. This can be easier and make the overall repair process go faster.
Restoring the Scratch and Brown Coats
Structo–lite is most commonly used to replace damaged scratch and brown coat. This product is made by USG (United States Gypsum) and can be found at most home improvement stores.
Use a tub to mix the structo–lite with water to a thick mud–like consistency. This product does not set quickly so it can be worked for a couple of hours or more before it becomes too stiff to use. Mix the whole bag at once for large repairs. For small repairs mix only what will fill the damaged area. Use a mud pan to work the structo–lite onto the wall. Collect the mud onto the rim of the pan and transfer it to the wall from there. For larger spaces the mixing tub itself can be used to transfer more mud quickly. For ceilings use a plaster hawk to hold the wet plaster and a trowel to push it into the lath.
Apply the structo–lite over the lath, forcing it into the gaps between the wood strips. Start at a corner or from the floor and push the mud from the pan into the lath. Apply an initial coat of mud to create the key and then apply another lightly over it to build up the brown coat. Build up a coat that fills the void in the damaged plaster to just below the surface of the surrounding wall or ceiling. Leave an eighth inch or less of wall surface to be filled with the white coat.
Use a straight edge, if necessary, to level the surface of the wet structo–lite with the surrounding wall or ceiling. Don't try to create a perfectly smooth finish in the surface of the mud. This coat should be a bit rough, the white coat will adhere better to a semi–rough surface. Allow the structo–lite to set overnight before proceeding.
Restoring the White Coat
When the structo–lite has set to a hard finish the white coat can be applied. There are a number of products available intended to replace damaged plaster white coat. For successful three coat plaster construction and repair, we recommend a setting–type joint compound like the one pictured here. This product will set to a smooth, hard plaster–like finish. It is sand–able and can be painted immediately. Setting–type joint compound is available at most building supply stores.
Plaster of paris is also available for repairing plaster white coat but we don't recommend using it. Real plaster of paris is very difficult to work with. First, it sets far too quickly for most do–it–yourselfers to use before it hardens. Second, it must be mixed precisely or cracks will form as it sets. Once set, it cannot be sanded. Lastly, plaster of paris cannot be painted right away. The alkali in the plaster must leach out before any finish can be applied, a process that takes at least 6 months.
Mix the setting compound a little at a time. This product will harden quickly, so don't mix more than you can use right away. Use a ratio of approximately 1 part water to 2 parts powdered compound. Try to produce a mud that will hold its shape when held upside down on the joint knife. Add more powder or water as needed to mix the right consistency. See Skim Coating Techniques for more on mixing and handling mud.
Apply the setting compound over the brown coat. Try to achieve an even coat that levels the damaged area with the surrounding surface. Use a straight edge as, described above, if necessary to keep the wall level. Don't try to smooth the surface of the first coat of mud. It will require a couple more coats to build a finished wall surface. Allow the first coat to set for about an hour before applying the next.
When the first white coat has set use the joint knife, in an upward stroke, to shave off any ridges or other protrusions in the surface. Alternately, coarse 80 grit sandpaper will work to smooth the surface before applying a second coat.
Mix another batch of setting compound and apply a thick coat over the the first. Use a straight edge if necessary to level the surface. In most cases, this coat will be rough like the first one. Allow it to set and remove any protrusions with the joint knife or sandpaper.
Mix and apply as many skim coats of mud as required to smooth out the finished surface. When the final white coat has set and dried, sand the surface smooth with medium 120 grit sandpaper.
