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Q. My drywall sub's crew members ran out of mud before they were able to finish the third coat of a recent project. But when the local lumberyard delivered three buckets of the familiar green-labeled compound, the two tapers refused to use it and instead drove nearly an hour round-trip to pick up the kind they preferred, claiming it was much easier to spread and sand. I wasn't aware that there was such a difference. Is there? Should different types of mud be used for different — i.e., first, second, third — coats?

A.Myron Ferguson, a drywall contractor in Broadalbin, N.Y., and the author of Drywall: Professional Techniques for Walls and Ceilings, responds: Companies that manufacture joint compounds actually offer a wide range of products, including taping and topping compounds as well as regular all-purpose compounds.

Taping compounds [1] are formulated to have excellent bonding strength and crack resistance for embedding paper tape. Topping compounds [2] that are formulated for fill and finish coats are lighter than taping compounds and not as strong, but they are less likely to shrink and are easier to apply and sand. All-purpose compounds ([3]) are the most versatile and can be used for embedding paper tape as well as for the finish coats; this is what you'll find at most lumberyards and home centers, and most likely is what your lumberyard stocks.

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Credit: USG Corp

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Credit: USG Corp

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Credit: USG Corp

As you've discovered, even all-purpose compounds are now available in different weights, each with different working characteristics. Many tapers will use regular all-purpose compound to bed their tape to take advantage of its strength, but it's common practice to then switch to a lightweight all-purpose compound for the fill and finish coats, since the lighter-weight compounds are a little easier to sand, and shrink less.

To avoid having to use two different products, some tapers prefer the midweight all-purpose compounds, which sand more easily than regular joint compound but are also suitable for bedding paper tape. This is probably the compound preferred by your tapers, who understood that mixing and matching compounds with different sanding properties can adversely affect the final result.

Most of these drying-type compounds (which have to air-dry before another coat can be applied) come in both ready-mixed and dry formulations. But don't confuse these powdered versions with the setting-type compounds [4] that come in powder form and are mixed with water. Since setting compounds dry chemically rather than through evaporation, they can be coated over when they set, even if they aren't completely dry. A setting compound is generally considered to be a stronger compound; it's the one that fiberglass mesh tape is typically embedded in to give the tape the extra strength it needs. But because most setting compounds are difficult to sand, they are primarily used for tape embedding and fill coats only.

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Credit: USG Corp