How to Prepare a Scratch Coat
By Nathan Giffin of Vertical Artisans
Scratch coats are particularly important in the world of vertical decorative concrete. I have even said a scratch coat makes or breaks the job. I say this because, in vertical decorative concrete, contractors must lay up a lot more than a regular plaster application. Most of the time we are laying up over 1” of material and even up to 3” – 4” of material. This requires a good rough surface with plenty of aggressive texture to help hold the material firm during the application process. If you have a very smooth scratch coat or brown coat, there is nothing to help with the weight of the mix. You are completely relying on the chemical bond of the products to hold fast to the wall. With an aggressive scratch coat, not only do you have a chemical bond but you also have a mechanical bond and this is key for thick applications of vertical decorative concrete mixes.
Scratch coats can be applied by hand, trowel, or sprayer. The most important aspect regardless of the method of application is that the surface is extremely rough when vertical decorative concrete is to be applied 1” or more. This is achieved by using a scarifier rake when the material is very soft.
Scratch Coats Require Proper Preparation
There is a big difference in preparing a cement wall vs. a drywall or stud wall with plywood.
Let’s start with interior surface walls consisting of plywood or drywall. Neither type of wall is good at all for a cementitious application. Plywood expands and absorbs water to the point of actually flexing out past the two-by-fours on which it is attached too. This flexing creates a deflection of the material and leads to massive surface cracks in the finish product. Drywall is not much better. It does not flex out but rather just deteriorates and offers no structural support whatsoever. Both sub straights, when unprotected, can cause many problems.
4 steps to follow to protect common areas like drywall and plywood
- Secure the surface with a moisture vapor barrier or plastic sheeting. This can be a house wrap or similar product as long as moisture cannot pass through. If you do not have access to this then common tar paper may also be used. This step only requires stapling. A common t-150 stapler will do just fine.
- There are two types of lath. Fiberglass Lath and Metal Lath. Metal lath is difficult to work with, particularly having many sharp edges from cutting it. Wear gloves and eye protection. Metal lath also rusts and will eventually cause problems in the structure or in the artwork of your project. Fiberglass lath does not rust and you can cut fiberglass lath with a pair of scissors. When applying lath to the wall surface you must al