I use the brush method myself, though usually cheap tooth brushes. Always helps the solution find those little gaps, and stick better if its agitated just a bit.
Heating the blade also really helps the etch to take hold. You can use a blow dryer, or if its hot and sunny out leave it in the sun a bit. Ideally heating the solution would help too, but that is more a factor with weaker acids, like vinegar.
As for concentrations, I use powdered FeCL and mix my own solution. I normally cut it to a 50/50 solution. If its too strong, then the blade builds a thin layer of oxides extremely quick eg. turns black, but I stress its a thin layer, and ironically the quick build up of oxides prevents further etching as the etchant will sit on top of the oxide layer and not go further in. Anyways, try different acids out, as different blades sometimes react better to different acids. I usually switch between lemon juice and FeCL. Lemon juice may surprise you if you havent tried it, as it is a very strong etchant, but sometimes tends for a more whitish color. I use it for initial cleaning etches, particularly if the blade is rusty (it can eat the rust without leaving large oxide deposits like FeCL). FeCL tends to be more black, and I really only use it if lemon juice doesnt give me the right color. Naval Jelly gives a grayish look. Etc... experiment.
I also recommend multiple short etches (let the blade soak 2-5 minutes), followed by light oxide removal (I usually remove with 2500 grit paper lubricated with a fragrant oil). Sometimes one etch is just perfect, but if not multiple etches are safer than one real long soak as it allows you to fix things in case something goes wrong. It also allows you to adjust the coloring, darker or light as your taste.
If you are really worried about damaging an inlay, or plated fittings, you can always seal them with a little clear lacquer or epoxy (crazy glue works). Easily removed with acetone.
Hmm...oh yeah dont forget make sure it is absolutely grease free. I normally wash the blade with dishwashing soap, then wipe it down with Isopropyl.
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