Coffee grinders are really useful for creating colour cosmetics and powdered products like powdered cleansers. You will need a dedicated, DIY-only coffee grinder—don’t try to make your kitchen one do double-duty! It’s pretty much impossible to get a coffee grinder clean enough that you won’t end up with shimmery or sudsy coffee.
I like blade grinders for DIYing; no need for a fancy burr one. My favourite grinder is one from Krups, but I also have some from Braun that have been great.
To clean your coffee grinder I recommend running some rice through it and then dusting it out with a dry brush. If needed, wipe out with a damp paper towel. Resist the urge to try to clean it like a blender by running hot water and soap through it—coffee grinders aren’t designed for wet things and that’s a great way to destroy it (I’ve done it!). You can wash the lid with hot water and dish detergent.
When grinding things up in a coffee grinder I highly recommend sandwiching a piece of cling film/plastic wrap between the lid and the grinding dish. You’ll still be able to run the grinder, but the cling film will remove the lid from the working area of the grinder, which is brilliant for making smaller batches of things.