How to Prepare Joint Compound

This story shows how to ready joint compound for application to drywall.

Intro

Setting-type compounds are an excellent choice for the first coat. Even with the relatively thick application needed to fill cracks and embed tape, you won't have to wait for evaporation before you move to the second coat. This can be an enormous advantage when you're working in humid weather. In addition, setting compounds provide more strength to the joints.

But setting compounds can be cumbersome to mix, and they have a restricted working time. Once the compound starts to set, you must react quickly to clean your tools and trash any unused material.

Premixed compounds are also often called ready to use. But more accurately, they are nearly ready to use. At least once a day, redistribute the moisture within the bucket with a mud masher. Otherwise the water migrates to the top, leaving the compound at the bottom of the bucket too stiff for easy spreading.

Keep the bucket of premixed compound tightly sealed when you aren't removing material. A snug lid keeps the moisture from evaporating while excluding dust and other contaminants from the bucket.

Checklist

Time
5 or fewer minutes per batch

Tools
Mixing bucket, kitchen scale, measuring containers, 1/2-inch drill with mixing paddle, mud masher (for premixed compounds)

Skills
Measuring and mixing

Prep
All drywall panels are hung, scrap material removed, floor is clean, drywall knifes, mud pan, and tape are at hand

Materials
Setting-type compound, drinking-grade water

Applying Compound

Before you apply the first coat of compound, install all metal or vinyl corner beads. Inspect the drywall by pushing each panel firmly in several locations. If there's any movement or sound, drive additional fasteners. You'll also need to fill any gaps over 1/4 inch and repair any damage that occurred during installation.

First coat (use setting-type compound or all-purpose ready-mixed compound)

-- Apply self-adhesive fiberglass tape to tapered seams, butt seams, and inside corners. (If you use paper tape, you'll embed it in the first coat of compound.)
-- Apply compound to tapered seams.
-- Apply compound to butt seams.
-- Apply compound to outside corners.
-- Apply compound over fasteners.

Second coat (use setting-type compound or all-purpose ready-mixed compound)

-- Apply compound in same sequence as for first coat

Third coat (use all-purpose or topping ready-mixed compound, or an easy-sand setting-type compound)

-- Apply compound in same sequence as for first coat

Setting vs. Ready-Mixed Compounds

Setting-type and ready-mixed joint compounds produce similar results but achieve them in different ways. Understanding these processes will help you select the right compound, or combination of products, for your job.

Ready-mixed joint compound hardens as the water in it evaporates. You cannot apply a second coat until the first is thoroughly dry. With an all-purpose compound, you usually have to allow overnight drying for the relatively thick initial coat. If you try to accelerate drying by applying heat or pointing a fan directly at it, you invite trouble. Under those circumstances, the surface can dry before moisture escapes from below, and a crack is the usual result.

A setting-type compound is a powder that you add to water. The addition of moisture begins an irreversible chemical reaction that causes one of the ingredients, plaster of Paris, to harden. The precise proportions of other components in the compound determine how long the mix stays workable. Grade 20 compound, for example, has a total working time of 10-20 minutes. Once the chemical reaction reaches a certain point, the compound becomes progressively difficult to spread until it's unworkable. Adding more water to the mix will not extend working time.

The setting time in the chart tells you the approximate time required until the compound has set enough to be recoated. For grade 20, you could apply a second coat about 20 minutes after you started mixing the first batch.

Fast-setting compounds are a great choice for prefilling or patches, but they can be difficult to manage for large-scale work. If you choose a compound with a longer setting time, you'll have more working time for each batch but still have a setting time that's fast enough for you to apply two or more coats in a single day.

If you utilize setting-type compound for the first and second coats, you take advantage of their speed and strength. Use a ready-mixed topping compound for the third coat, and you'll enjoy its advantages of easy workability and sanding. Using a pair of products is called a two-compound system.

Step 1

If you're a first-time user of setting-type compound, carefully measure the ingredients until you acquire a feel for the correct consistency. Start with a clean mixing bucket, and pour in the amount of drinking-grade cool water you need for the batch size you're mixing. Weigh the compound, and gently sprinkle it into the water to avoid splashing.

Step 2

Chuck a mixing paddle into your drill, and stir the compound until you've removed all lumps. Keep the drill at a low speed; you want to stir, not whip air bubbles into the mix. Hold the bucket between your feet so it doesn't spin. Scrape the sides and bottom of the bucket with a piece of scrap wood to distribute any pockets of powder. Clean the mixer immediately after mixing so the compound doesn't harden on it.

Step 3: Prepping Ready-Mix Compound

Loosen ready-mixed joint compound with a manual mud masher. Moisture tends to migrate upward in ready-mixed containers; sometimes you'll even see a layer of water on the top. A few up and down strokes will help return the product to a uniform consistency. Avoid overmixing or power mixers because you'll whip bubbles into the compound, and they cause finishing problems. Discard the compound if there's any mold growth or it has an unpleasant odor.

Step 4

You've achieved the right consistency when the joint compound doesn't slump under its own weight or readily slide off a taping knife held at a 45-degree angle.


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