First, let me say I have never tried Caswell's zinc kit.
(just never got around to buying it)
I have been acid copper and nickel plating for a longggg time and have been using Caswell's "CopyChrome" for about (2) years.
That being said (and perhaps none of this applies), but here goes:
1. Anode to cathode (part) distance:
It is important to keep the parts at least 3" away (or further) from the anode to prevent burning and excessive current draw. Get too close and they will "fizz" and burn.
2. Agitation:
This is key. I use a combination of coarse (large) air bubbles and a device that rotates the parts in the solution.
I was having extreme issues with copper plating ("sandy" looking parts, burnt areas, extreme build-up) until I started using LARGE (1'4" dia.) air bubbles to agitate the solution (even though I was using the automatic part rotator). Forget about using those airstones....they just don't work too well.
Now the copper comes out a super smooth pink color (as before it looked like copper colored sand granules).
A note here: exactly the same set-up as before when I was having trouble with the only addition of the coarse air bubbles.
3. "Fizzing" action on part(s):
If I see any "fizzing" action the part is ruined!
The coarse air bubbles keeps the tiny hydrogen bubbles off of the surface and allows fresh solution to come in contact with the part. Additionally, too much current or anode too close to part will cause the part to "fizz".
4. CC/CV lab power supplies:
I have one. 0-18V @10 amp.
Believe it or not, I set the voltage to the recommended setting (or slightly lower) and let the part draw what it wants. I have hardly ever set the current limiter adjustment and all my parts come out fantastic. I keep an eye on the digital amp meter as copper will tend do draw more current then nickel/CopyChrome.
Nickel: 2.25V
Copper: 1.5 to 2V
CopyChrome: 4.5V
Now, from what I am reading in your post, I belive it can be rectified by #1 thru #3.
#4 may be due to the fact that my parts are always 3" or more from the anodes.
Give it another try and let me know.
See some of my plating at:
http://users.adelphia.net/%7Epatpawz/geo/plating.htm
George W.