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I've had good luck using oven cleaner, which is just sodium hydroxide, or lye, in water, with some sort of chemical to help it stick to the surface. Oven cleaner is just easier and safer to handle - but you probably could just mix up lye and paint it on. Because it is water soluble, it can be washed off easily using a pressure washer. Getting the residue off of whatever you use is critical to getting good primer adhesion. The problem with paint stripper is that is is solvent based, and takes solvent to remove it's residue. Did you ever strip a piece of wood, and then go back with solvent and a rag? There's a lot of residue that comes off till it's clean. That's why the wood stripper makers sell an after strip cleaner. Even though wood's obviously more porout than steel, you still need to get the residue off before painting. Another thing I found is that you can't just wire brush paint off and then prime. Try wiping down a shiny surface that you've just brushed with a wire wheel, with a rag dipped in solvent - it's amazing how much surface residue comes off. Particularly so with cast. I think a fast turning wire wheel burnishes the residue onto the surface. If the primer is going to stick as well as it should, the surface has to be physically clean, not just shiny.
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