The best way to get rid of these stains is with a simple pickle that
someone put out there a long time ago. Some wizard (Peter Rowe? Tim
McCreight?) developed a recipe for peroxide pickle that works
wonders. Formula: 3/4 C. drugstore strength hydrogen peroxide, 1/4
C. grocery store strength white vinegar. Get it hot but not boiling.
Pickle with this, makes it all pink; then put it in the Sparex
pickle, pink gone. Repeat as necessary. Good for light firescale
too. If you're coming up with the stains immediately after etching,
before soldering, this means you need to neutralize the metal better
after etching. Use clear household ammonia in a 1:8 ratio with
water. Let the piece set for 15 minutes and scrub well with a
toothbrush. Rinse in water. Should solve your problems.