How to Mix Stucco: 8 Tips

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. Stucco is typically applied in 4 coats; the dash coat, the scratch coat, the brown coat and the finish coat.

Step 1 - Mixing the Dash Coat

The dash coat is spattered onto a concrete or masonry substrate to strengthen the bond with the scratch coat. A dash coat consists of 1 part by volume of Type I Portland cement and 2 parts sand. Just enough water is added to provide a workable mix.

Step 2 - Mixing the Scratch Coat

The scratch coat is 1 part Type I Portland cement, ¼ part hydrated lime and 2 to 4 parts sand, with enough water added to give the consistency of a slurry. The scratch coat is applied to a thickness of ½ 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 - Mixing 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 3 to 5 parts sand. The brown coat is troweled on evenly to a thickness of 3/8 inch to present a level surface for the application of the finish coat.

Step 4 - Mixing the Finish Coat

The finish coat is mixed in approximately the same proportions as the base coats (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. 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 inch in thickness and can be worked, either by hand or machine, into any desired texture.  

Step 5 - 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. Increasing the ratio of cement used will cause the finished stucco to have greater strength but will also make it susceptible to shrinkage and cracking. The sand must be free from loam or dirt and should be passed through a sieve before being used to remove any small stones.