Using the same spray-paint I'd use if I was finishing it an opaque color. Up here in french-Canada I use basically anything I can find from the hardware store (Tremclad by Rustoleum works perfectly).
Here are a few examples, I don't etch all my pedals but I do have fun with some of them:
1- For this one (devi ever Hyperion) I put some masking tape strips on, painted, removed tape, etched, painted and sanded:
2- This is a tubescreamer. Flat black coat, drew with a screw, dipped in acid:
finished:
3- Green russian muff:
4- 1590a latching footswitch:
5- TS808 + SHO inabox:
6- tri-wreck:
7- BATW Pharaoh:
None of these look professional by any stretch of the imagination but they look good enough to me so...