One of the pop-out window frames was media blasted and all of the small parts that were epoxy primed earlier in the week were primed with high-build urethane primer. The purpose of the epoxy primer is to seal out moisture and form a protective shell around the metal, however it is not easily sand-able as it quickly clogs sandpaper. The purpose of the high-build urethane primer is to fill small surface imperfections to create a smooth surface, the urethane primer is readily sand-able and meant for this purpose. The right picture isn’t great but it shows how this works, the let side of the part has been sanded and the holes left by the rust pits are much shallower, the right side is unsanded and the holes left by the rust pits are still very deep. (compare to the un-blasted part at far left) Further sanding and additional coats of high-build primer (if needed) will yield a perfectly flat surface.