I am getting an incredible amount of blackish scum deposit on
the metal that has to be brushed off with a toothbrush.
Blackish scum suggests to me a chemical contamination, a reaction
with the iron. Is it possible you’re getting any kind of ammonia in
the solution? Ammonia causes this reaction. I suppose it would
possible with other chemicals accidentally introduced to cause a
similar reaction. Are you using a glass or plastic container to etch
in? I wonder if some of your set up materials might be having a
chemical interaction with your solution? I wonder if ascetic acid as
by-product of tape adhesive or butyl latex caulk might cause this.
The wave pattern (or ripples), from experience, is generally caused
from too strong a solution. When etching large open areas, you need to
cut your solution by about 15%-20% more water. Your etch will be
slower, but more uniform, with less undercutting. If the etch is going
too slow, you can either try the spray or rocker method.
Also, when using an aquarium bubbler, remove the airstone (most
decompose and react with the solution), and protect the piece by
placing the end of the hose under a glass sheet, and then suspend your
piece above the glass sheet. This allows the solution to be agitated,
but you won’t get scour marks from the increased action directly
against your plate. Actually, I pierce the hose with a large needle
down the length of the last 8", and plug up the end. This allows a
pretty uniform agitation and oxygenation, without a large
concentration in any one area. Oxygen combined with the ferric,
increases the etching action. Therefore, when you have a large
concentration of air, such as at the end of a bubbler, the solution
has a tendency to etch more rapidly on the end of the plate closest to
the air, causing scour marks, which look like pits or striations.
I hope this helps to solve your problem.