Use the same etching soln over and over. As long as you don't contaminate it somehow, it's still good.
It doesn't seem to eat away at pvc. The tube I have it in has been in there for years.
Maybe it goes 'bad', but I haven't experienced that yet.
Bottom pvc end cap glued into place. Top cap just sits on the top.
I use a weak soln, about 2% by my estimation. Drop the bbl in and take it right back out again and immedietly flush the stuff off under running water.
Then begin gently carding under the running water with x-fine steel wool.
Some people 'etch & card' once after 2 or more rust, boil & carding cycles that build color. Then again multiple rust, boil and carding cycles to build more color,,then one etch cycle w/carding and so on.
(The Boiling if you want Black and White as opposed to Brown & White finish)
Others, me included, etch & steelwool card after each rust, boil and carding.