A 'thank you' to all the people who made suggestions to help fix my C41 issues. I processed another film on Sunday and they came out great.
The things I did differently:
1. Used the correct center column for the tank this time! It sounds like a stupid error, but having two tanks almost the same size stored in the same box is a recipe for errors...you have been warned!
2. I used the Tetenal C41 kit, but added a 2% acetic acid stop between the colour dev and blix stages. I used a couple of washes (water at 38C) after the stop, about a minute each with constant agitation.
3. I shook the blix vigorously for about 30-40 seconds just before I used it.
4. I increased the blix time by 50% as recommended on here, which made a total of 9 minutes.
5. Immersed the film in the stabilizer in the open tank and gently agitated it manually, rather than running it on the Jobo and creating a foam party!
The end result is better shadow detail, richer colours, no colour casts or streaks and improved contrast.
Really enjoying my new home dev hobby
