High-pressure water blasting

Water jetting is a fine thing for removing rust, scale, and old coatings before welding. The big advantage: it's purely mechanical, so no thermal stress on the material, and you don't get distortion or microstructural changes in the weld zone. You just have to be careful not to get too close or stay too long in one spot. Otherwise, you'll mill the surface and make it rough or create microcracks that you don't need when welding. Start with moderate pressure and increase slowly until the coating comes off cleanly.