rinse it in cold water spread on sheet in sun to dry, when dry put it in a dark glass or plastic container let set a few days then give it the sniff test.
"red dust" is usually RUST from the inside of the "can".., you did say "can".. ?
If the powder is outgassing NOx enough to create rust within the can, rinsing the powder will not do anything as the stabilizer within the powder mix has been consumed.
ROLE OF DIPHENYLAMINE AS A STABILIZER IN PROPELLANTS;
ANALYTICAL CHEMISTRY OF DIPHENYLAMINE IN PROPELLANTS
Nitrocellulose-base propellants are essentially unstable materials
that decompose on aging with the evolution of oxides of nitrogen. The
decomposition is autocatalytic and can lead to failure of the ammunition or disastrous explosions.
Heat, as you can see in the report, will age gunpowder
Section from the Propellant Management Guide:
Stabilizers are chemical ingredients added to propellant at time of manufacture to
decrease the rate of propellant degradation and reduce the probability of auto ignition during its expected useful life.
As nitrocellulose-based propellants decompose, they release nitrogen oxides. If the nitrogen oxides are left free to react in the propellant, they can react with the nitrate ester, causing further decomposition and additional release of nitrogen oxides. The reaction between the nitrate ester and the nitrogen oxides is exothermic (i.e., the reaction produces heat). Heat increases the rate of propellant decomposition. More importantly, the exothermic nature of the reaction creates a problem if sufficient heat is generated to initiate combustion. Chemical additives, referred to as stabilizers, are added to propellant formulations to react with free nitrogen oxides to prevent their attack on the nitrate esters in the propellant. The stabilizers are scavengers that act rather like sponges, and once they become “saturated” they are no longer able to remove nitrogen oxides from the propellant. Self-heating of the propellant can occur unabated at the “saturation” point without the ameliorating effect of the stabilizer. Once begun, the self-heating may become sufficient to cause auto ignition.
It is best practice to discard old gunpowder with red dust. Might be overly conservative but old gunpowder does not burn evenly and has caused blowups.