I use the 5-1-1 mix rather than the gritty mix so my advice might not be very helpful. Gritty mix users on the list might have better advice.

I think you have a couple of options. One option is to screen individual ingredients and then rise the remaining dust off of them. Another option is to screen ingredients, mix your soil, and then flush out the fine material very well once your plant is in it. I'm not sure it makes much difference as long as the majority of fine dust is removed from the gritty mix.

As for storage there are a few options. You can store your gritty mix either as separate ingredients or already mixed. Either way is OK. I usually have a few gallons of mixed 5-1-1 on hand, minus the lime or CRF, in a large plastic storage bin. The other ingredients I keep in the original bags until I'm ready to mix them. If you have room to store bags of ingredients and don't want to buy plastic bins, then the original bags are fine. Plastic bins are good for keeping ingredients dry and neatly stacked.

For long-term storage I prefer to keep everything dry, so if you decide to rinse your ingredients to remove dust I would let them dry very well before placing them in a plastic storage container for storage.

Hope that helps.

TYG