The dip would be over the factory paint after a little surface preparation. Anything going on under the dip would also be going on under the factory paint if it could compromise the tank. So what difference does it make?. I touch-up or repaint my tanks all the time. If there is something wrong with the base coat, such as corrosion, all the layers will skin off the bubble. The first thing I do when I buy a used painted tank is to pull the valve and look it over, then skin the outside with a rough grit sandpaper looking for any paint to flake or deep scratches in the paint to leech some rust. Once that is done, it's primer and touch-up. Never had any problems with the place where they are hydro-ed. Decidedly, having my compressor and doing my inspections and maintenance comes in handy, but sometimes I have to travel with a few of them and have them visual ed at the site before filling. Never had a problem with that ether.
I say if you are going to dip it, try a piece of large PVC pipe first, coated with the primer you are going to use, just to see how the dip would look on the cylinder. I would sure hate to remove the dip time and time again from my tank until I got right. You can pitch the PVC. Be aware that in most cases the factory paint is polyurethane and will hold a compatible primer well. Once you get a primer bond, you can use an enamel or lacquer top coat.