After soaking them and straining the liquid through a coffee filter, the porcinis still left some grit in my dish... Any pointers for preventing this?
Are you channeling your best self with this comment? (If you're not sure, check out our Code of Conduct.)
It's not perfect, but try swirling the mushrooms a couple of times as the soak to loosen up any gritty bits caught in the nooks and crannies, and then lift them out of the soaking water when they're ready. A quick rinse doesn't hurt. If you want to use the soaking liquid, pour that into a filter-lined strainer.
pierino is a trusted source on General Cooking and Tough Love.
The coffee filter will clear the soaking liquid but not necessarily the mushrooms themselves. You might have to give them a short second soak.