I use a
Trinco bead blaster, just a basic unit, like their model 30. For the least effect on the surface, I use Mil Spec #13 glass bead, screen size 175/325(it's about like talcum powder), at 20 PSI. On hardened tool steel, this will reduce your size( on the EDM'd surface, not on ground surfaces) about .0001 total. It will improve your measured surface finish a couple of points. It will not round off your corners.
If you use coarser beads, or more pressure, to make the job go faster, it
will round the edges, as it will on softer steels. I have also used Scotch brite to clean up parts after EDM, either hand pads, or on a flap wheel. On super critical parts, I have in the past left a couple of thou stock allowance, and finish ground them after EDM. That may not be an option if the customer is supplying the parts.
I also use a Branson ultrasonic cleaner, but it will not take rust off. On parts that are not carbon steel, (stainless, Inconel, Hastelloy, etc) I use a mild acid wash (AC500). Again, rust is not the issue with them.
There are additives you can put in your dielectric to combat rust. There are some people who use them, and like them (Savan is one example). Others will tell you they have had bad experience with them. I don't like putting anything but water in my tank
. You should make sure your conductivity is correct, check your sensor to make sure it is clean, run your make up fill hose through a seperate deionizing tank before you put it in the machine. Use a fine mesh filter (under 5 micron).
Newer machines have anti-electroloysis power supplies to help minimize this problem. They help, but are not perfect. Try to take the parts out as soon as they are done, blow them dry, and do whatever surface treatment you're going to use, right away. On a submerged machine, leave the tank filled if the part is going to sit in the machine, draining the tank only makes it worse.
I have seen people recommend a light dusting of WD40, or similar items, or coating them with Dykem. I have used the Dykem method on sinker parts where I wanted to protect an adjacent surface from "spatter". You need to be sure you don't end up electrically isolating the part from grounding.
Change to a material that is more rust resistant
.
Cheers!
Roy Solomon