If you have access to a steam pressure washer, you can more than likely get the majority of that light surface rust off before coating. You can still coat with MPHD with a light layer or surface rust without issue. The MPHD will eliminate exposure to the elements/moisture and not allow it to get any worse, in other words, the rust will not continue to spread once the MPHD is sprayed on. But it would still be a good idea to try to get as much off before coating as possible.
