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 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 dry wall 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 cementious applications. 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.
Here are the steps to follow to protect common areas like drywall and plywood.
1.) Secure the surface with a moisture vapor barrier or a 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.
2.) 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 can 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 always fasten to the studs in the wall if going over drywall. If you are applying lath over plywood you still want to target the studs but anywhere is really good. You can use large staple guns, roofing nails, plastic headed nails and screws. There are also recommended fastening systems available from the producers of the fiberglass lath. Ie. www.SpiderLath.com
3.) When installing the lath make sure you overlap the pieces by several inches. This insures full coverage and strength of the scratch coat products.
4.) When doing decorative ends or open ends do not allow the lath to go all the way to the edge or end. You should keep the lath back 1” so to avoid exposing the lath after final applications of mud are applied.
Brick, Concrete and Masonry Walls
These types of walls simply need to be inspected for dirt and debris. Power washing a surface is a good way to ensure a clean surface. Some concrete surfaces may already be very rough and able to hold a vertical application; however, it is still good to cover the surface with a fresh coat to avoid exposing the actual base while carving a design. For instance, if you are covering a brick wall and you do not use a scratch coat, you can expose the brick and joint lines with an aggressive carve or design. The client will then be able to see the red brick or mortar joints amidst your art work. Bonding agents are also good for scratch coats over these types of surfaces. Flexy-Bo is a great bonding agent when considering concrete and brick surfaces.
Foam Core Construction and Foam Sub Straights
When using foam substrates, I always us Structure Coat from Walttools. This mix is specifically designed to fully adhere to the foam and ensure a maximum bond. One applied properly you have a very solid seamless surface. This mix reaches 12,000 psi in and offers great flexural strength. When applying with a Mortar Sprayer there is no need to scarify the surface. The pattern that is achieved with the mortar sprayer is sufficient for an excellent scratch coat. Once the Structure Coat is dried the surface is ready for the application of your vertical decorative concrete.
Preferred Products for Vertical Decorative Concrete
Walttools Tru-Pac X – For Scratch Coats and Carving / Stamping Coats
For non-foam scratch coats the best product is actually the same as the carving or stamping coats. Tru-Pac X is a fantastic scratch coat material reaching 5000 psi and chemically bonding to itself in later applications.
Walttools Flexi-Bo Bonder
TI Flexy-Bo is a concentrated re-emulsifiable co-polymer solution designed as a primer for overlayments such as Walttools Over-Eze,Tru-Pac X & V vertical overlay, and standard underlayments.