Not really sure what you mean by rinsing off dust I'm afraid.
How do you know the astringent flavors are from dust?
How are you measuring mash pH? When are you measuring it?
I don't know, just something I haven't tried. The spreadsheet calculates the pH. I just trust it from what others say. Trying not to spend $130 on a quality pH meter. Do you use a meter?
Sorry, rinsing of the dust would be simply running dark grains under the faucet for a minute in a strainer of some kind.