I’d love to see a comparison of both. 
Sand would probably give a nicer finish, but I don’t know if it’d work using the dryer method.
Edit: How do finishes like these on titanium fare with torching? It’d be sweet to get the best of both worlds, but idk how it’d end up.
I don’t think sand will do much outside of a blasting cabinet.
What if it was the bright gravel AND sand? Like the sand wouldn’t have enough mass to affect a finish, but with the gravel in there pushing it around … Maybe? Idfk I ain’t no sign-tist.
A combination could work… I’d really like to see it done 
Also how important is this part? I was just gonna throw it in the normal load…
It might melt your plastic container. Dryers get pretty hot!!
I was planning on using an old nalgene bottle, so… I think it would be fine
Ladies Love My Cool Aquatic Substrate
I got the smallest kind, because I feel no-water and smaller is gonna work best. Let’s SCIENCE THIS
Ingredients:
I’ll wrap this in a couple layers of towels and throw in in the dryer on “no heat, tumble only” tomorrow but for now I’m manually shaking the crap out of it for a bit to make sure the bottle is secure and to see what happens.
Already got like a post apocalyptic look to it. Nice.
hmmm, wonder if this would work with plastics
Seems like if it works with Ti, it would gouge plastic. you make NEED to use sand in the case of plastic to prevent deep scoring. But again,
It is running now! I wrapped the container in two towels with a bit of tape and it is surprisingly not that loud in the dryer. On tumble only no heat of course.
EDIT: ok, this experiment is kind of a fail. It tumbled for 100 minutes and mostly it… polished… the yo-yo. So these rocks are definitely too small, my theory was wrong. Sand would be even worse presumably
Phase two!
State before (kinda polished based on the last run, very small rocks was not a good idea, sand is out of the question)
Larger acquarium rocks
edit: first hour in the dryer… definitely looking more scratchy, but nowhere near the first post. I’m going to try lightly moistening the rocks and see if that helps for the next hour of tumblin’
Next hour with damp rocks
Rim areas looking pretty good, actually!
The problem is the rocks aren’t really getting to the cups, or closer in to the response area… really at all.
I’m thinking using an assembled yo-yo here isn’t the best way because the shape prevents the rocks from really getting all over. So for phase three, I’m gonna do each half individually in the container, no silicone pads, no axle, just the halves… for one hour each half.
I don’t feel there’s going to be any damage to silicone pad areas or axle hole if they are left unprotected, the rocks are wayyy too big to get into those crevices.
I saw a hot glue idea which I think would work great for masking off the pad area and axle hole but seems potentially difficult to get out of axle hole. I agree your substrate shouldn’t impact axle hole and honestly roughing up the response pad area shouldn’t hurt anything.
Thanks for sharing this experiment Jeff it’s really interesting to see the results!
Final result
My protips™ are
You will end up with a “worn denim jeans” kinda burnished look if that’s what you are shooting for. Less shiny, more textured.
* Colored rocks will also work but you’ll be shedding a lot of color between runs 
3 questions-
Thanks for the Science!
