We are considering forming an assembly and then painting it rather than painting the individual parts before assembling. My problem is that the assembly is made up of a mild steel box section and aluminum castings. The castings will be pre-treated by anodizing or conversion coating as they are readily available treatments in this area. Once assembled, the assembly will pass through an iron phosphate spray pretreatment prior to being painted. Am I asking for trouble spraying over the already pre-treated aluminum? D.W.
Normally, pretreatments are not applied over pretreatments. They should be applied direct to metal. They must react with the metal and will not react with phosphates and chromates to form a bond to the metal. Some pretreatments contain chemical stages that are aggressive enough to remove the existing surface pretreatment and will do just that. In which case you may apply a conversion coating that is inferior to the original one.
In your case, the chemicals normally used in the iron phosphatizing process should not harm the anodizing or conversion coating on the aluminum castings. The cleaner stage will degrease the aluminum parts, and the iron phosphate stage will do no harm. If you are in doubt, try a representative aluminum sample or scrap parts. Actually, you have nothing to lose by conducting this exercise.