Homebrew PCBs

The objective in any etching system is to replace the spent etchant at the 
surface of the board with fresh.  The speed of the etchant moving doesn't 
contribute to the etch rate other than to bring fresh etchant to the 
surface.  Friction between the etchant and the board surface makes this 
quite difficult.  This is why spray etching is the best - if you have enough 
of the right spray nozzles to produce a uniform spray pattern.  In a spray 
etch 1 oz copper foil will etch in a little over a minute.  If you look 
inside you'll see why.  The etchant is rushing over the board surface at 
about a thousand feet per minute.  In a bubbler system your lucky to get to 
50 feet per minute.  In an ultrasonic cleaner there is almost no movement so 
all the etchant does is sit there and shake.

I remember trying reverse electroplating as an etch method in an attempt to 
minimize waste treatment and equipment costs.  The circuit board was used as 
the anode and a sheet of stainless steel used to collect the copper.  It 
didn't work very well and was way too slow for production.  A lot of things 
have been tried, but the critical factor is always getting fresh etchant to 
the board surface.