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.
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.