I'd clean, degrease (brake cleaner, acetone, wax and grease remover, or simple green), then scuff with a red 3m pad (available at any hardware or auto store), then use the metal ready, allow it to fully dry, then paint. 

The metal ready I believe is an acid etch which microscopically abraids the surface so the paint can stick better. 

-craig

On Nov 8, 2013, at 9:44 AM, David Bjorkman <ddbjorkman@VERIZON.NET> wrote:

> Hi All;
> I am about to do some POR15 application.  I have 2 very different surfaces.  I found, to my great surprise, that when the bus was painted, the sliding door mechanism cover was not removed and I have a layer of rust over that area, including the rail the roller runs in.  The other surface is a new rear (standard, not Rocky Mtn) bumper.  I want to put the POR 15 on the inside of the bumper before installation.
> My question is should I use Metal Ready on either one of these surfaces before applying the POR?  As a matter of face, how should I prep both of these surfaces before applying the POR?  I am would like to start this project today, so any input would be appreciated. 
>  
> Thanks;
> Dave B.