For this kind of very fine powder, weather sealing and simple rain covers usually are not enough. The safest approach is an airtight enclosure, ideally one designed to be waterproof for immersion. That can be a rigid underwater housing or a sealed soft waterproof bag/housing made for shallow water use.

Why: the powder behaves like fine airborne dust and can get past normal seals. A waterproof enclosure is useful because you can rinse or spray the powder off the outside before opening it.

Practical tips:

• Use an airtight waterproof housing rather than just wrapping the camera loosely in plastic.
• Clean and dry the outside thoroughly before breaking the seal.
• If you are not fully confident in the seal, avoid zooming because some zoom lenses “breathe” and move air in and out.
• A prime lens, or a zoom with internal zoom/focus, is a safer choice than a lens that extends.

So yes: for powder events, think airtight more than merely weatherproof.