There are various products in the market which, when mixed with joint compounds, claim to improve various performance attributes of joint compounds.
USG does not endorse the use of additives in USG joint compounds for the following reasons:
An additive can adversely affect the unique balance of the many performance attributes each joint compound has been formulated to provide.
It is virtually impossible for an additive manufacturer to know if their additive is compatible at all times with all of USG's product formulations.
USG has no control over the manufacture and quality of additives manufactured by others.
Note - For taping and finishing joints, water can be added as necessary. USG recommends adding water in increments of a half-pint to avoid over thinning.
See USG Technical Information Sheet J1716 for more information.