What are some different types of Drywall Mud?
Last time you visited the local hardware store or big box home improvement store looking for drywall mud you may have been puzzled by the variety. Do all joint compounds work the same or should you use a certain type of mud for certain applications? Let's discuss the two basic categories of mud and then we can break down the choices within these two categories. This article is not meant as a review of any brand or recommendation of a certain product line, rather this is a general discussion of the options out there.
Quick-setting or "hot" mud.
Quick-setting drywall mud comes in a powdered form usually in a plastic lined paper bag. The plastic-lining helps to keep moisture out and preserve freshness. Keeping the powdered mud dry is critical. Once water comes in contact with hot mud it begins a chemical reaction that will cause the mud to harden. Once quickset mud hardens, thats it, you cannot reuse it. Even small parts of the compound that have hardened within a mostly good bag can cause the entire bag to be unusable.
Hot mud hardens by means of a chemical reaction. Manufacturers mix different amounts of hardening agents into the material causing it to set up quicker or slower. Hot mud is sold according to the time it takes to harden, for example, 5-minute, 20-minute, 45-minute, 90-minute or higher. Actual working time is slightly less than the time listed. In other words, 45-minute hot mud may begin to set up in 30-35 minutes.
It is important to remember that the time associated with different quick-setting drywall compounds is the hardening time, not dry time. For example, 45-minute hot mud may harden within 45 minutes but it may take several hours to completely dry depending on temperature and humidity levels. When using hot mud you will notice it turn from dark grey to white over time. The mud cannot be considered completely dry until it turns completely white in appearance. Unlike traditional drywall compound, it will not shrink as it dries, so it is possible to add additional coats of hot mud before it has completely dried.
When to use "hot" mud
Quick-setting drywall mud is great for small patch jobs where you don't want to wait 24 hours before applying a second coat. It is even used on large jobs to bed in the tape where the pros want to apply a first coat of all purpose mud over tape all within the first day. This can take a job from being a four day process due to drying time, and get it done in three days.
Many pros feel that quick-setting mud provides a stronger base coat and more protection against cracking in some situations. Therefore some use it as a first coat when finishing corner bead.
Most pros agree that if mesh tape is used, only quick-setting mud should be used. They feel that mesh tape does not provide a strong a bond and therefore needs the added strength of "hot" mud. In fact some manufacturers also recommend using quick-setting compound when choosing mesh tape.
Quick-setting mud is rarely used as a finish coat, mostly because it is more difficult to sand than "light-weight all purpose" mud. Even when hot mud is used for coating patches, a skim coat of light-weight all purpose mud often is used over earlier coats of quick-setting mud.
There are several brands of quick-setting drywall mud. The pros all have their preferred brands and different reasons for each. Two popular brands are USG SHEETROCK® brand EZ-sand and Gold Bond® brand Pro Form® quick-setting drywall mud.
Why is it called "hot" mud?
Why is it called "hot" mud? We are not completely convinced as to the reason but there are a couple possibilities. As the mud hardens, the chemical reaction, creates a small amount of heat. It is mostly indiscernible however, the heat may be why some people call it hot mud. On the other hand this may simply be an idiomatic description emphasizing that you have to move quickly once mixed because it will harden up on you if you don't finish within its specified time.
Pre-mixed drywall mud
The other main category of drywall mud can be described as pre-mixed. It is what most people consider traditional drywall mud. It is sold in either 4.5 gallon buckets or in boxes containing a plastic bag of mud. Pre-mixed drywall mud is, as the name describes, pre-mixed and ready to use.
It is almost always thinned slightly with water before using. The amount of water used depends on the application. When bedding in tape, a thinner consistency may be preferred. When using automatic taping tools, a thin consistency may be needed to allow smooth flow through the drywall mud pump and tools. If the mud is thinned too much it will be difficult to spread on the walls and will shrink back too much. Texture mud is the thinnest consistency in relation to other applications. It takes time and training to understand what consistency of mud should be used with each application.
When shopping for drywall mud you will come across different types of pre-mixed mud. The three most commonly used are "all purpose", "light-weight all-purpose" and "topping". There are of course several other types of joint compound available however these are the most common.
All purpose joint compound
All purpose joint compound is as it says made for all applications. It tends to have more gluing agents which is helpful when bedding in seams. It is sometimes used to first coat seams and corner beads. Less often it is used as a finish coat because it is more difficult to sand than "lightweight all purpose" joint compounds. However some professionals prefer it for finish coat for the very reason that it creates a harder finish that is more difficult to sand. They feel that this may provide a more durable product.
Light-weight All Purpose joint compound
Light-weight all-purpose mud is as it says made for all applications yet is lighter in weight than all purpose mud. Many pros believe that light-weight joint compound contains less gluing agents and therefore is inferior when taping seams. They prefer to use all purpose or taping joint compounds. Light-weight compound is often used for first and second coat of seams and corner bead. It is most always used as a finish coat because of its ease of sanding.
Topping compound is used less often then the first two products discussed. It is used as a top coat for seams and corner bead. It is not recommended to be used for taping joints. The color of topping is whiter than all purpose and even whiter than light-weight all purpose. It is many times used for texture applications.
Are joint compounds compatible with one another?
Generally speaking, they are compatible with one another. Joint compounds can be used over each other but most manufacturers do not recommend mixing the compounds together. It is always the best practice to refer to the manufacturer's specific recommendations.