You can also paint some nail polish over anything you don't want etched.
Then use acetate or nail polish remover to cut the polish off.
Or, if you are going to stonewash the hardware after the etch, just leave it on and that will knock it off, while saving what's under the polish.
See this thread
http://www.bladeforums.com/forums/s...pyderco-PPT-Outlander?p=10625856#post10625856
I etched and stonewashed every steel part on those two projects.
As far as time and mixture, I used the ferric full strength. Then using nitrate gloves, just "massaged" the stuff, then water rinsed. If it wasn't dark enough, back in the acid. Took maybe 3-4 minutes. trick is, to keep it moving in the bath. Hanging it from a thread or what ever is OK, but there's no need as long as you swish and turn the parts, AND WEAR GLOVES. And clean the parts 1st with soapy water and rinse, while wearing gloves. Any oil spots (fingerprints, etc) will not allow a consistent finish!
Make sure you rinse the finished parts really well, then immerse them in a warm bath of water (about a quart), and about 3 TABLESPOONS of baking soda (NOT baking powder). Turn and swish the parts and let sit for several minutes.
The reason to do this is because the ferric RUSTS the metal (like bluing) and it will continue even after rinsing. Hard to put an edge on when 2 hours later, that fresh edge is "rusty" The soda will help slow that down a LOT. Even with that, you will see some edge darkening for a few days.
Make no mistake, acid bath and stonewash is DAMAGING the blade. A controlled damage, but make sure you want to do this as it takes a LOT of work, and takes off a lot of steel, to get back to a polished finish, even with just a stonewash.
This is my experience, and I have done 6-8 knives like this.
http://www.bladeforums.com/forums/showthread.php/929524-My-Spyderco-Mods
And take note, each steel type will give a different result. S30V will not be exactly like VG-10, or 440C, etc.
Stonewashing instructions would take another big post, at least MY way of doing it would.