The objective of the finisher is to complete the order on time and meet all quality criteria. History is a good place to start. Were there problems on prior runs? A good shop will have documented records, including quantities and yields of prior problems. Examine those records well before running the production order. Many times, this activity will lead you to items to check prior to running the order. For example, if prior records indicate high fallout due to a contaminate on the raw part that the pretreatment system could not remove, then an inspection and perhaps a trial run should proceed the production run.
The problem is that the trial run must be fast and not disruptive to the normal finishing process. To accommodate this, use a few "sacrifice" parts and coat them in whatever coating the line is currently processing. If the parts turn out well, without signs of the past defect, chances are high that the production order will run well.
You also can enhance the pretreatment preparation in order to assess the acceptability of the parts in question. Take one or two parts and mechanically abrade them using sandpaper or blasting media. Run two more parts with a solvent wipe prior to pretreatment. Add a few parts "as is" and run your trial. Assess the trial parts visually and also conduct physical testing, such as scribe test or pencil hardness. Do the three different test parts behave differently? This is an excellent predictor of the success of the production run.
If the past problem has been due to spraying conditions, you can use different testing methods. For example, if prior parts were coated heavy or light, go over the criteria with the sprayers and let them spray a few parts on the normal rack used to see if their technique has solved the problem. You may loose some oven time, but it's better than having an oven full of unacceptable parts.
If color match was the issue, spray a part and cure it in the normal cure oven and assess the results. Measure the part using the same criteria as you would a production part, either visually or spectrophotometer measurement.
If prior problems were due to unacceptable conditions in the raw parts coming in, such as rust or other contaminates not removed by a commercial wash system, carefully inspect the raw parts for the condition in question. Another technique for trial is to wash a few parts and run them (without coating) through the cure oven, then rewash the parts and proceed with the trial run. Always run a few "normal process" parts at the same time so that you can compare the results. If the "normals" failed and the preheated parts passed, you might want to notify your customer and suggest a process deviation using the preheat routine to burn off any lodged contaminates. If you take the time to show the customer the trial prior to running production parts, chances are strong that they will agree to extra compensation in order to get good parts. If the customer does not want the additional process, then a written deviation from the customer is a good idea.
Using this pilot approach takes a little more time. If it is well thought out, you can accomplish the test without delaying the order for any length of time. If there is absolutely no time to conduct any tests, then at least go over the history of problems with key people, such as coaters or inspectors. This approach is better than simply running the parts and hoping for the best.