USES
Asphaltum and shellac stopouts stop out completely, offering a high degree of control but only flat tones; an "all or nothing" situation.  Permeable grounds offer a field of activity between "all" and "nothing".  They are applied to the plate in a film, or "layer" that has varying thicknesses, usually on top of an aquatint, (though they can also be applied directly to the bare metal plate) and where they're thin the etchant breaks through quickly and etches a tone and where they're thicker the etchant breaks through more slowly and creates less tone. If you first apply an aquatint and then apply the white ground on top of it, the aquatint creates a tone and the white ground creates lights and darks in that tone; as any stopping out of an aquatint will do. If you apply it directly to the metal with no aquatint, you get more pitting, flat biting, and textural effects as opposed to the tonal effects you get if you use an aquatint. The thickness of the ground has to be coordinated with the planned time in the etchant to achieve the desired range of tones, and once the darkest tones desired in a given area have been achieved, if more etching is to be done on the plate, the area has to be stopped out with asphaltum or some additional permeable ground to keep it from etching further and getting too dark.  Permeable grounds open up a world of looser, more spontaneous and painterly effects to etchers who otherwise are more or less confined to techniques that tend to be mechanically rigid and labor-intensive.

APPLICATION
Application of White Ground is discussed in detail in Ruth Leaf's book.  Basically, it is thinned with water to whatever consistency is desired and applied with a brush, sponge, rag, or whatever is within the scope of an etcher's imagination.  There is a learning curve, and an etcher should be thoroughly familiar with the way they behave (i.e. do some test plates) before investing a lot of time in using them to create imagery on a plate.  I have used them for years on zinc etched with nitric acid and on copper etched with ferric chloride, and that is the extent of  my experience with them.  They tend to be more permeable on zinc/nitric than on copper/ferric chloride, but generally behave the same otherwise.  Most are water soluble (though they tend to leave residual white grease on brushes and other implements) until they have been immersed in acid or ferric chloride for at least 10 or 20 seconds or more, after which they are waterproof, which is handy for rinsing off plates between step bites.  If the plate (white ground) has been in the acid/ferric 30 seconds or less, water-rinse very carefully to avoid lifting the white ground. In the less-than-a-minute range it's best to rinse by dipping the plate in a tray of rinse-water, rather than hitting it with pressurized water from a tap or sprayer. They can be removed with mineral spirits after etching is complete.  They dry to a soft greasy coating, which can be further manipulated by scratching or scraping with a wooden point or the corner of a piece of cardboard, blended with cotton swabs or stumps.  I use them over aquatints made with Graphic Chemical's brown powdered rosin, since I've found this type of rosin to be more durable and resistant to abrasion, or to being loosened or dissolved by solvents or dislodged by manipulations with materials like permeable grounds than the lighter amber rosin that comes in chunks and is ground to use for aquatints.  It's also more visible on the plate.

They can be used by themselves in thick or watery consistencies to create imagery, or can be combined with other stopouts such as airbrushed asphaltum or spray enamel to soften or modify their effects.  They can be used for semi-controlled textural effects by laying down a field of thin watery permeable ground and spraying or spattering wet-in-wet incompatible liquids like watery asphaltum dissolved in turpentine or mineral spirits into them, or spritzing them with kerosene from a spray bottle.  Stand by with a hair-dryer to dry them and stop the action when you see something you like.  They can be used as a base for sticking other materials to the plate for use as a resist, for instance you can get a good rock texture by laying down a very thin (watery) field of wet permeable ground and sprinkling particles of crunched up lump rosin in to it and letting it dry, then etching. I usually do this on top of a rosin/box aquatint which gives a tonal effect, but a more textural effect could be achieved by doing it without one.

A "hard" stopout like asphaltum, shellac, or rosin varnish only stops out tone.  A "permeable" stopout, those being discussed here, both stop out and create tone.  They stop it out (where they're thick) until they break down (where they're thin) and when they break down they create tone.  In actual practice however it's best to think of them all as just "stopouts"; things that prevent etching from happening.  Take a "step-biting" approach rather than a "one-shot" approach. So when step-biting an aquatint if you want white, apply them before any etching is done.  If you want tone in a given area, etch to the lightest tone you want in  that area and then stop it out and etch to the next darker tone.  With a "hard" stopout, you get a series of flat tones. With a "permeable" stopout you get much more complex results but thinking of it in this way can make the complexity (discussed in the next two paragraphs) more manageable.

ETCHING
The actual etching part of the process, as always when you're doing etchings, is a highly critical guessing game.  The question is, how dark do you want it to be and how long do you leave it in the solution to have it be that dark.  This guess (timing) is of necessity based entirely on the etcher's previous experience with regard to three primary factors which all interact with each other and which (at least in actual practice) defy accurate description. These are a. the strength of the etchant, b. the character of the aquatint that's on the plate, and c. the amount of permeable ground on the plate.  Every timing decision is at least a three dimensional judgment call.  I usually approach it by first deciding where in the image I want what percentage of tone.  I might have a line proof or working/line tracing of the image at that point, and write on it where each % of tone goes. Then I decide what amount of time in the acid ("maximum etch time") is going to give me a 100% black.  This decision is based on how fine or coarse and how open or dense the aquatint is.  A fine aquatint will max out to black in the acid and start to break down much sooner than a coarser one will, and an aquatint whose particles are more sparse and segregated on the plate will max out sooner than one whose particles are so dense that they're all starting to run together.  I shoot for an aquatint where the dots are still separate, but almost starting to fuse together.   Acid strength of course is critical to this too.  In practice, I maintain my acid (ferric chloride) at one fairly constant strength, and my aquatints are usually of one fairly constant description, so I assume about a 15 minute max. etch time on a warm day and a 20 or 30 minute max. etch time if it's a cold day or if the bath is a little old or if I'm worried about it being too light.

This "maximum etch time" which is a professional guess is the basis for timing charts which I have devised to break down the total  etch time in to 10% increments of grayscale.  It takes much less time to get an aquatint to darken from 10 to 20% than it does to get one to darken from say 80 to 90%.  These timing charts, of course, guarantee nothing.  They're only a way to break down a chunk of clock time into shades of gray that are approximately in the character of an even progression.  You start with a guess, and when you get to the end of the chart you look at the etched plate with the dots of rosin or whatever on it through a magnifying glass and say "well, that looks like it's about at 100%", or maybe you look at it when it's supposed to be at 80% and say "Oh my gosh! I'm starting to loose it!"  In which case you spatter it with an air brush or spray can and keep on etching.  If you get to the end of the chart and think it doesn't look very etched, you make another guess and extend the timings you have left, if any.  I usually try to err on the dark side.  At least that way you have something to scrape, sand, and burnish on, and it's easier to get precise tones with direct work on the plate than with theoretical timings.