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 in drywall mud. Do all joint compounds do the same thing or should you use a certain type of mud for certain applications? Let's discuss the two basic categories 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 meant to be 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 the contents' 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 has hardened within a mostly good bag can cause the entire bag to be unusable.
This type of mud sets by chemical reaction. Manufacturers will mix different amounts of hardening agents into the material which will cause it to set up quicker or slower. They sell the stuff according to hardening time, so you can buy 5-minute, 20-minute, 45-minute, 90-minute or higher depending on how much working time you need. Actual working time may be slightly lower than the number listed. In other words, 45-minute hot mud may begin to set up in 30-35 minutes.
The time measurement is referred to as hardening time rather than drying time because although the mud may harden in the specified time frame, it does not dry within that time frame. When using hot mud you may notice a dark grey color that turns white over time. The mud ccannot be considered completely dry until it turns completely white in appearance. Therefore it can be recoated while still wet because it has already hardened. Once hard it will not shrink up or move as it dries.
Why is it called "hot" mud? Perhaps it is because of the heat released during the chemical reaction as the mud hardens. 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.
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 coat 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 they will use it as a first coat when finishing corner bead. Most pros agree that only quick-setting mud should be used with mesh tape. 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.
Pre-mixed drywall mud
The other main category of drywall mud can be described as pre-mixed. It is what most people consider your 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 preferable to thin the mud 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 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 is as it says made for all applications. It tends to have more glue like agents which is helpful when bedding in tpaper tape on the 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 is as it says made for all applications yet is lighter in weight than all purpose mud. Many pros believe that light-weight joint compounds contain less gluing agents and therefore are 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 much 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. Some pros have had success mixing some of the compounds when using for various applications, however it is always the best practice to refer to the manufacturer's recommendations.