99.9% of any successful coating requires excellent pretreatment. The cleaner the material, the better success you will have achieving adhesion, smoothness and longevity.
Powder coatings can provide improved performance over liquids when applied to a properly pretreated part. Solvent-borne paints are usually more forgiving of organic soils left on the work piece by sub-par cleaning. Because powder does not have solvents, you need to make sure the washer does a good cleaning job. This is just good operational practice and is not an unusual requirement. Iron phosphate is the most frequently encountered pretreatment used with powder coatings. However, if the highest level of performance is required, zinc phosphate will work admirably with powder as well.