How to Mix Stucco Layers
This article will give you some helpful advice on how to properly mix stucco. The various layers that make up a stucco exterior wall finish contain basically the same ingredients, but are mixed at different ratios. This material is typically applied in four coats: the dash coat, the scratch coat, the brown coat, and the finish coat. Follow the steps below for mixing each layer and get this amazing exterior finish for your home.
Step 1 - Mix the Dash Coat
The dash coat is spattered onto a concrete or masonry substrate to strengthen the bond with the scratch coat. It consists of one part by volume of Type I Portland cement and two parts sand. Only add just enough water to get a workable mix.
Step 2 - Mix the Scratch Coat
The scratch coat is formed using one part Type I Portland cement, 1/4 part hydrated lime, and two to four parts sand, with enough water added to give the consistency of a slurry. The scratch coat is applied to a thickness of half an inch and should completely surround the lath. Before drying, it is scored or grooved to create a strong mechanical bond with the subsequent coat. Together with the lath it creates a rigid base for the application of the brown coat.
Step 3 - Mix the Brown Coat
The scratch coat and the brown coat have essentially the same ingredients at the same ratios, except that the brown coat has three to five parts sand. It is troweled onto the surface evenly to a thickness of 3/8 of an inch to present a level surface for the application of the finish coat.
Step 4 - Mix the Finish Coat
The finish coat is mixed in approximately the same proportions as the base (scratch and brown coats), but uses white Portland cement plus mineral oxide pigments to add color. Do not add more than 10 pounds of pigment per bag of cement. Also, be aware of the ingredients used to mix the finish coat and in what ratio so that you can duplicate the exact color in a new mix. The finish coat is only about 1/16 of an inch in thickness and can be worked, either by hand or machine, into any desired texture.
Additional Helpful Tips
Only mix enough stucco that you can apply within one hour. If the mix becomes stiff, add more water until you get a more plastic, fluid mix. Excessive amounts of either hydrated lime or sand will weaken the mix, and while increasing the ratio of cement used will cause the finished stucco to have greater strength, it will also be susceptible to shrinkage and cracking. The sand in your solution must be free from loam or dirt and should be passed through a sieve before being used to remove any small stones.