On Jon Lybrook's recommendation I eventually purchased the Boxcar Press brush. Dianne Longley talks about any flat soft bristle brush being usable for washout. After constructing my own using taped together bristle paint brushes from Home Depot, I decided to buy the Boxcar brush. At 4" x 8", it fits neatly in the palm of my hand. The soft bristles present a flat broad washout surface to the plate. A plate washer I saw at Kala Institute during a workshop had a large flat brush with bristles about 1" long if I recall correctly.
As Jon and others have suggested, I line the bottom of a plastic tray with a 60 mil magnetic sheet to hold the steel-backed KM73 plate still during washout. I fill the tray with tap water to just near the top of the Boxcar brush as it rests against the bottom. The water is about 70 deg. F. I do not at this time worry about the temperature of the water greatly. I keep my darkroom at about 70 deg. F also.
After first exposing the plate to the positive, and then to the aquatint screen (talcing and brushing the plate before each exposure), I place the plate into the water and allow it to settle centered on the magnetic sheet.
I fire off a two minute timer - my washout for a plate about 8" x 10" is two minutes. Following Longley's suggested wash out procedure in her book Printmaking with Photopolymer Plates, I brush the plate in a circular motion, moving from one side of the plate to the other. I think I am making 3" to 4" elliptical motions with my hand during washout. I make sure to extend the brushing about two inches beyond the plate edge when starting to ensure I wash the entire plate.
I do not put much pressure on the brush. I start by letting the weight of the brush apply the pressure, and add a bit more with my hand. The brush moves across the plate easily - though you will quickly feel friction from the "etched" areas of the plate and the bristles find a hold in the depressions that will later hold the ink. After doing this several times I think that fine bristles are probably important for washing out plates made with a fine aquatint screen.
When I reach the other edge, again ensuring I brush two inches past the plate edge, if necessary I shift the brush down, and continue brushing with a circular motion, overlapping the strokes to the other end of the plate.
If the plate is long you may repeat this motion, reversing at the edge of the plate one more time.
Jon's site mentioned keeping a stroke count to ensure a uniform wash out. In trading e-mails with him he mentioned he is a bit more relaxed about the stroke count. That said, here is a conundrum. You really want to uniformly process the plates each and every time to get consistent results. If you use different size plates (for different size prints) this becomes interesting. I have been using 8.5" x 10" plates (cut down from larger originals) during my calibration and testing. I'm not sure what I will do when I tackle larger plates. This might be the basis for the warning on the Boxcar site to only use the brush for plates up to 9" x 12".
When I reach the end of the plate I rotate the brush 90 degrees and begin brushing along the other axis of the plate. I vary the orientation of the brush to ensure lightly scrubbing the plate in a fairly random variation. Truth be told, I then rotate the brush 45 degrees and attack the plate (gently) on a diagonal axis.
And then I repeat until the two minutes are up. Note I have read many recommendations for time to wash out. The time recommended is less for relief printing, more for fine halftone work. I do not pre-soak. And I use a soft brush with gentle pressure.
I lift the plate out of the water and, using a garden hose sprayer head set to a broad spray pattern (not a jet) on the end of a hose, I wash the plate down with a fair bit of water pressure. I make several passes over the plate with the sprayer.
I let the plate drip dry for a few seconds before laying it down on a sheet of blue shop paper towel for drying.
The polymer plate is delicate at this stage. The polymer is only partially hardened. I take care to not touch the surface of the plate with my bare hands, nor scratch it with fingernails, the hard handle of the brush, or knock it into anything as I move it around. There's no repairing it once scratched.