I don't know what kind of stuff the powder is made of but it is corrosive and should be washed off the engine as soon as practical. It is not recommended to run an engine covered in powder and definitely not a good idea to start the engine if the powder can be ingested!
As for the "keep cranking to suck the fire in" theory, it doesn't always work!
It depends a lot on the design of the carb/airbox...