I don’t think sanding woud work. I’d try using a grind stone on a drill and then redo it.
If you want it smaller and lighter get a small light wooden block that’s hollow and drill a whole into it and then do the whole splash process again and there, you have it, a really nice counterweight with splash on it.
oh it’s wood? don’t use the grind stone then…
You can make a counterweight really out of anything. I just prefer a counterweight made out of wood. 
It’s plastic.