Phil,
Its quite simple the hardest part is painting the plate.
I bought a cheap used laser printer off ebay.In my case a Samsung ML-1860. It had half the original toner cartridge still in (worked great) and later found out that cheap compatable toner cartridges dont work very well.
I printed out the reverse image onto mylar drafting film using the best quality and blackest print settings.
I then placed it over the brass plate which was cleaned and polished before hand with alcohol or acetone and ironed the mylar on a hot setting .
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Carefully and slowly peeled off the mylar before it cooled too much and it leaves the black toner as a mask on the brass plate.If all goes well it should look like this.
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I then coated the back with a couple of coats of shellac,quick drying acrylic varnish will do,just to stop etching on the back of the plate.
I then prepared a simple warm bath of ferric chloride in an old icecream tub. I didnt take much notice of the strength of the solution but probably about 25gms in 1/2 litre of water.
Then just put it in the bath and gently tilted the tub occasionally to keep te solution a bit agitated.
You can use a very soft brush to brush the surface but after a while the toner becomes sensitive to abrasion.
Heres a different plate in the bath.
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Thats basically it ,clean off the toner with a dish cleaner scruber (like the scotchbrite pads).