Yes you would have to apply a coat or rust proofing emulsion on top.

Many make the mistake of not converting the rust surface first and the rust continues to spread under the paint - no matter how good the paint is. The zinc will also help the paint to bond better. This is a reason factory paint tends to last - the metal is dipped into zink phosphate first. You may notice if you get a chip in your new car, the rust will not start for a long time even though metal may be exposed, the zinc is protecting the metal.

A quality restoration should have a rust conversion done first. This is also true for undercoating an older car.

If you are restoring an area with bad rust, you could also pre-clean the metal with muriatic acid, neutralize and wash, then the zink.

If you just sand the metal, the rusting process will start right away, even though you don't see it yet. Humidity in the air, combined with oxygen. Ask your body shop - if they don't know what you are talking about - find someone who does.

soon...