Well I'm a rubber stamp manufacturer!
I actually manufacture rubber, not polymer stamps. There isn't much difference in the products these days, except when you wish to use quick drying inks often, or for long periods.
Natural rubber is obviously the best, except it is anything except natural, well it's as natural as the rubber tyres on your cars.
Almost all inks in rubber stamp land I know of, are one of two types. Water based and solvent (or oil) based.
Basically the ink is water with a dye in it. The water is the carrier, the dye is the colour.
When you stamp a piece of paper the carrier dries into the substrate, which is the paper, leaving the dye on top. You have a rubber stamp inpression which looks good but may run when the image gets wet. Not always but often.
With non porous material, stamping gets a bit iffy, to say the least. Non porous material is any surface that is sealed from the outside world.
Resin Coated (RC) paper used in bucket loads of darkrooms and AFAIK, in all commercial colour paper processes, is a non porous surface.
What is required is an ink with a high solvent content, generally these inks are oil based. The carrier in this case is the solvent base, which by the way, evaporates very quickly when it hits the atmosphere.
Basically when the impression is made, the solvent, depending on it's makeup, evaporates reasonably quickly and leaves the pigment dye on the surface. Basically I liken it to a type of house paint in the way it works and sticks to anything it touches.
I myself don't think (in this day and age) that rubber stamps are that great for the back of RC prints. That said, I do use rubber stamps for the back of all of my RC prints.
When it comes to stamp pads for the use of quick drying inks, the best in the world is made in Japan. It's an Artline product and the pad size is 65x103mm of which about 55x93mm is actually really usable. This pad comes with it's own airtight snap lid with a rubber seal all around.
The pad is called, Pad HI-SEAL EAGU-3 This is the smaller of two available. As it's so expensive I wouldn't worry about the larger version. My Guestimation is that this pad would probably retail in the USA between $25 - $60 USD
The ink designed for these pads is also called HI-SEAL and it comes in a 55ml glass bottle. The screw cap has an applicator attached, which is a bit of wood and a brush at the end. Like glue bottles we have here. This ink is called, ink HI-SEAL ESSG-1.
Superior rubber stamp and seal company in the USA, is a reputable marking device manufacturer, which is what they are called in the USA. You may wish to look at Millenium Marking as well.
The biggest manufacturer of inks for all sorts of applications like this, is in Germany. The company is Noris inks. Noris make almost every conceivable ink, they really do it well.
If you wish to rejuvenate a crudded up ink pad which is full of quick drying ink, try Isopropyl Alcohol. This will be heaps cheaper than any manufacturers product and I would guess, probably do as good, if not better job. It's what I use!
Mick.