Ross:
Got any photos of your parts? That would help a lot.
What I'm about to tell you contradicts the instructions, but it works for me:
1) You need more than 25ma/sq-in. (300/12). You'll need at least 50, 80-100 works best. See my
Current Density Test Results
2) if you're using the 1.5v/300ma wall wart that comes in the kit, it's impossible to know how much current it's actually delivering unless you measure it with a meter, and it's difficult to get repeatable results from. See my
Wall-Wart Characteristics
3) you should dip the part in a mild acid solution after plating/before chromating. The electrolyte leaves a brown film on the part after plating, which might interfere with chromating, (but my guess is that you don't have a good plate due to insufficient current.) See some
Sample Parts
4) you don't need a lot of agitation. How much depends on how many sq-in of parts you're plating. The more parts you plate, the more gassing occurs, and the more agitation you'll need. A simple aquarium pump is all I use to circulate the electrolyte. It works well for up to @ 50-sq-in. If I have bigger parts, I put a second pump in. Some
Photos & Videos of Agitation
5) How strong is the acid you're using before plating? If the acid is too strong, you may well be dissolving the steel, which will turn the parts black. You only need a mild solution, 2-5%.
6) Do NOT use SP degreaser between the acid dip & plating bath.
I would recommend this procedure (rinse between each step):
a) thoroughly pre-clean and de-rust the parts.
b) if the parts have a pre-existing zinc/cad plate, acid dip until all fizzing stops
c) hot SP degreaser for a few minutes
d) acid dip for 30-60 seconds
e) plating bath
f) acid dip for a few seconds, just long enough to remove the discoloration. You will be able to SEE the brightening action.
g) chromate
This has been working well for me. You can see my results in the link below.