Also, staining will occur were the planter metal comes in direct contact with the surface the planter sits on. If you are placing your planter box on grass, lawn or dirt than there is nothing to worry about. Or if you never plan to move the planter you will never see the marks it leaves underneath the floor. But if you plant to move the planter and do not want rust marks you should make sure the planter metal does not come in direct contact with the surface that can be stained. For our planters this can be achieved by placing a plastic strips on the channel feet/legs the planter sits on. Another, solution is to place the metal planter box onto casters. Placing the planter on casters will eliminate direct contact and allow you to easily move the heavy planter.
Generally, If you cannot stand the ideal of the smallest amount of rust on your deck or patio Corten planters are probably not ideal for your application so consider other metal planter options like Stainless Steel or Powder coated aluminum.