this is the process for Electro Etching by Richard Caine

This procedure is electro plating in reverse.

The following steps detail the procedure.

1) Make up a bucket of salt solution. Common cooking salt is fine.
2) Thoroughly clean the steel from which the valve is made. Use soap
and water to remove the bulk of any preserving oil coating and finish
off with a good solvent.
3) Spray paint the steel with aerosol paint both sides to a thin even
layer. The paint forms the etch mask and must adhere extremely well to
the metal. It took me a few tests to find a paint that worked well. I
use a spray primer. Leave it a few days to cure, or it will peel during
etching.
4) On one side of the steel score through the paint the outline of the
valve. Use a very sharp blade and make the thinnest possible line. Wide
scores make erratic edges when etched. If the piece of steel is large
enough you can do multiple valves. I never do less than 4 at any one
time.
5) Get out the car battery charger set to 12V, connect the negative to
scrap steel and the positive to the piece to be etched. Drop these in
opposite sides of the bucket with the scored side of the valve metal
facing the scrap on the negative.
6) Switch on the charger and ensure that the current is below .5A per
valve. If there is too much current just add more water to the salt
solution. If you have a variable power supply, lower the voltage. The
less the current the slower the etch, but the finer the etch. The
negative will effervesce hydrogen so it's a good idea to do this
outside or in good ventilation. You should soon see the outline of the
valve score become black.
7) After 15 or 20 minutes the valve should gently press out of the
sheet. If not all of it has been cut through put it back for a few more
minutes. I check it every 5 minutes or so.
8) Clean the valve with solvent to remove the paint and then clean up
the edges with emery paper. You now have a perfectly flat valve with no
machining burs which will seal well against the engines valve head.

Common problems

The main problem is the paint peeling during the etch process, which
leads to ragged valve edges. This is usually due to one of three
things.

1) Paint not allowed to cure long enough.
2) Steel not clean enough when sprayed. Oil still on the surface.
3) Paint not suitable. Some paints just don't work.. I tried a few
before I found a primer that gave consistently good results.