Which of the following should you never do with FFFFg blackpowder?

Using FFFFg blackpowder as a powder charge is not advisable because it is extremely fine and intended primarily for specific applications, such as in the priming of black powder firearms or as a propellant for small-scale applications like in toy cannons or special effects. Its fine granulation increases the burn rate significantly compared to coarser blackpowder grades, which can lead to a dangerously rapid combustion if used as a main charge in a firearm. This could result in an unsafe situation, potentially causing the firearm to malfunction or even explode upon firing.

In contrast, there are other considerations associated with the other options. For instance, using FFFFg for priming is acceptable and common practice, as this fine powder can ignite easily and effectively. Storing it in a humid environment can lead to deterioration and clumping, which can compromise its performance and safety. Mixing it with other types of blackpowder can also be problematic, as it may alter the burn characteristics and performance unpredictably, risking safety during use.