The problem is that the salt spray test results are 200 hours and we are having difficulty achieving 250 hours consistently. Should we change the pre-treatment for ZN phosphate? The previous pre-treatment was iron-phosphate. Any other suggestions?
Before you change the pretreatment you should be absolutely confident in your quality control. Is your plant insuring that the process tanks are properly titrated? Have you confirmed that the final rinse stage is free of contaminants? Do you know that the alkaline degreaser is removing all the oil and grease? What do you know about the cleanliness of the shot blasted surface, other than that it is "near white"? Are you controlling the blast profile? Are you failing the salt spray test because the powder coating is not adequately covering the peaks of the profile?
Unless you have answers to my suggested questions, you are not ready to make a decision to change processes. Even if you do convert to zinc phosphate, which from an environmental standpoint would be a step backwards, unless you know more about the quality of the process you might simply find yourself with the same problems.