por-15 satin black. one coat and you're done. no primer, unless you are anal about using primers. also, you might want to completely strip the paint from both sides of the floor before proceeding. you might find other spots. por-15 will stop the rust from progressing when you apply it to bare metal, but if you apply it over existing paint you won't be stopping anything - just delaying it. the rust will continue to grow under the original paint and some time in the future will bubble up under the por-15. that might lead you to believe that por-15 is less than perfect, but in actuality it was the process of removing the old paint that was less than perfect.
also, while por-15 is an excellent paint, i don't think you can paint over it. so you might want to consider if you would want the undercarriage to be any other color before you start applying the satin black.