If you have a metal roof, you no doubt understand the problems these roofs can develop with oxidation or rust buildup. More often than not, some oxidation on the roof will not cause a problem. Most roofs are made of a durable enough metal to withstand the elements and the rust they will naturally encounter over time. But if you plan on painting or restoring the roof or if you can easily see the oxidation, you may want to remove that oxidation and have the metal looking new again.
If possible, sandblast your roof; the results will most likely satisfy you. Sandblasting, the same technique used to strip rust and paint off automobiles when restoring them, works just as well on metal roofing. The only real drawbacks with sandblasting involve the time and money required to make use of it. Rent a sandblasting machine at a specialty shop, and carry out the proper preparation. Wear the proper protective clothing, then simply blast away any oxidation, leaving a clean piece of metal. Take note, however, that if the rust has eaten through the metal at all, sandblasting it away will only open the hole and require replacement of the roofing panel.
Modern chemical treatments provide a relatively safe method to use both for you and the environment; if your roof has mild oxidation is mild and you're willing to pay for the chemicals, consider this quick option. Essentially a liquid version of sandblasting, chemical rust treatments remove the oxidation buildup from the metal fairly quickly and easily. Again, this brings the added risk of exposing any holes or imperfections where rust may accrued over time. Another drawback of the chemical method: It can become quite expensive. Buy the chemicals involved, often available only in specialty stores, by the gallon for a premium. If you can obtain these chemicals, spray them onto the affected area and allow them to sit for a few hours. After that, wash it away with water; with it will go the remains of the dissolved rust.
For the most accessible and cheapest way to deal with oxidation on a metal roof, use special spray to "convert" it to a permanent solid that you can then paint over. Although it works very well and quickly, only use this method if you plan to repaint or restore the roof. Look for the spray in most hardware stores; it looks similar to standard spray paint. In fact, most spray paint companies make some form of conversion spray as part of their product line. Apply the spray to the oxidized areas just as you would paint, then allow it to set and dry. When dry, the rust will become a solid protective surface that you can treat or paint just like the rest of the roofing surface. It won't weather the elements for long periods, so don't use this method without treating it afterward.