Our last Zodiac had the RI numbers stenciled on either side of the front and they were quite legible for over 8 years, uncovered and unprotected in the Caribbean tropical sun, 24/7/365. I didn't put them on, but they seemed to be a regular enamel paint, probably spray.
After 40 odd years of owning inflatables (never a RIB) I have found that to paint or glue one, you must sand the sheen off the material and make absolutely positive that the areas to be worked on are perfectly clean, and this means of fingerprints as well. We wash the area with acetone before applying the paint or epoxy glue, and we get 4 years or more from glue jobs (mostly the lifting tangs) and the paint gets twice that.
Since we've never had a breakdown of the material or a leak due to construction (like at the seams), I can only assume it is because we only buy the best quality inflatables, French built Zodiacs or in the old days, English built Avons. You can really see the difference in quality when the Zodiac Mark II Classic is sitting at a dinghy dock surrounded by Highfields, Achilles, WM et al.