I forgot to mention, The manganese phosphate is the dark gray or black looking parts. The zinc phosphate is a really light gray and is mostly used as a base for paint. The valve covers, air cleaner, and many parts were zinc phosphated originally before they painted them. I would assume that primer is just as good but I will have a few parts powder coated and primer will not survive the oven. The zinc phosphate will. Powder coat don’t have to have anything under it but if rust does start under it, it will grow under the powder coating and will all flake off. The zinc will prevent that from happening.