I have always prefered the NH3 H2O2 etch.  But as you found out,
it has a limited shelf life.  Because of this, (in a past life)
we would mix up a few drops at a time in a watch glass as needed,
and dip the sections in the watch glass.  In these volumes,
evaporation tended to keep the solution fresh.  And  when it all
evaporated, well, make up another 10 drop batch.  So we had a
small bottle of NH3 and one of H2O2 at the grinding table, with a
watch glass and paper towels.  One used q-tips, the others just
dipped in the watch glass.  

-- 
George Franck
Principal Product Engineer
Raytheon E-Systems
Falls Church Va

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