When doing Damascus Barrel finish , either Brown/White or Black(Blue)/White
I always used a very weak soln of only around 3% Ferric Chloride in Water .
This was used as a very quick dip into and then back out of process.
The rust browned or rust blued bbls then quickly rinsed under running water to clear the etch off the surface and at the very same time the surfaces were carded with clean 0000 steel wool.
I did this with every 'cycle'.
Some only did the etching cycle every few rustings.
The Black/White pattern requires the rusting to be boiled as well.
The Brown/White pattern does not.
I found using and etch soln much stronger than the 3% would destroy much of the color I was trying to build up.
The etching soln was loosening the color just built up on the Steel component and it being carded off left the 'white metal look. The softer Iron surfaces held the color better and lost less of it through the etching process.
The color pattern is left behind.
Many cycles are needed to build the final finish.
3 steps forward ,,1 or 2 back each time.
If you are mearly trying to 'etch' your way to a Damascus pattern and are not concerned with the traditional bbl color patterns,,then a stronger soln will get you there.
As above demonstrates,, a 1 to 4 etchant to water seems to work nicely.
That strong of a soln would remove all of any built up rust brown or blue color with each etching step done.
