The competitor says to rinse metal prep and let the surface rust for a few days before coating, should I do this with Mastercoat Metal Prep?

No. Always let Mastercoat Metal Prep dry and do not rinse, no matter what other brands' instructions say. Our Mastercoat Permanent Rust Sealers won't be affected by the dried acid leftover on the surface after the Metal Prep dries, and the zinc film left by our Metal Prep is great for adhesion purposes. It is still better to start as clean as possible with the surface; if you rinse the Metal Prep and let it rust again, there is no point in using Metal Prep in the first place.