Rule #1: Conducting a review of your cleaning and prepping process with a knowledgeable chemistry professional is the key to assuring your pretreatment success and avoiding frustration long-term.
When it comes to chemistry, consult a professional who SPECIALIZES in pretreatment chemistry. With that being said, do enough homework to understand the major issues of parts pretreatment before you call in a pro.
Rule #2: If you want to have consistently good results, you must set up a cleaning and pretreatment routine that will be sufficient for your WORST parts—not your best parts.
The most important step is to carefully examine the parts you want to clean. You have to be honest about just how clean or dirty your parts really are. Countless coating jobs have been sabotaged because the coater didn’t do an adequate job of prepping the part(s). This often happens because the coater only looks over a couple sample parts and then goes to work. Soil is anything that is on the surface of a part. Dirt, grease, oil, shavings, wood, waxes, metal oxides, release agents, you name it. If it’s on your part but not supposed to be, it’s a soil. For you to achieve a quality finish, all of the soil should be removed. It is important that you examine a large sample so you can feel confident that you know how much soil is actually on the parts you are going to be coating.
When it’s time to coat, slow down for a moment and look over the part(s) to see if they are still as clean as they were when you completed the pretreatment process. If they aren’t, work on them some more. This is an absolutely critical step on your road to getting top-notch finishes. It can be very tempting to say, “that’s close enough” and coat a part after some rust has popped back up or a grease stain isn’t totally removed. If you don’t give in to temptation, the chances of getting stuck with a re-do are much smaller.
Rule #3: If it’s organic soil it will probably need an alkaline based cleaner. If it’s non organic, it will probably need an acid.
This is only part of the consideration, though. Oils and greases need not only a highly alkaline-based cleaning solution but, since many industrial oils include types of wax, it is likely that the cleaning solution will need to be heated. Polymers, like silicone, will need an aggressive acid and may also need heat. Many soaps and dry lubricants typically need not only heat and a highly alkaline solution, but mechanical action, as well. Particulates, solid pieces of things like metal shavings, may need an acid/alkaline mix and vigorous mechanical action for full removal. Finally, oxides (rust, tarnish, etc.) will require a finely balanced acid to completely remove the oxide without damaging the surface of the part being cleaned.
Good luck, and remember there is no such thing as a part that is too clean.