The weathering process allows some of the metal's alkalinity to dissipate on its own. You'll still have to clean it thoroughly to help neutralize the alkalinity, as well as to remove any surface oil that had been applied to prevent white rust. Selecting the appropriate primer is one of the cardinal rules of surface prep for problem surfaces. To do so, you must know the general characteristics and chemical properties of the surface you're coating. Try as much as possible to determine what's currently covering a surface before you coat over it.
Some stains contain waxes, for example, and many cleaners contain waxes and silicones.
Stevens recommended wiping a section with a cloth dampened with thinner. Under ideal circumstances, you'd always know what's on the surface now. However, circumstances rarely are ideal, so ask questions. Then you can get a bottle of the product, see what it's made of, and determine whether or not you need to prime the surface. Redwood, cedar, and fir are especially prone to tannins, which are water-soluble dyes in the wood, bleeding through and discoloring topcoats.