Obviously the degradation is on the inside of the tube, so no amount of paint will do much good. If not too extensive, a patch or two might work for a while, but once the inside of the tube begins to degrade, you will be chasing leaks forever.
If you have the finances and you feel this dink is worth it, inflatable/liferaft service shops could help as mentioned above, but in my opinion, you'd be throwing good money after bad. Call this one a (hopefully) not too expensive lesson and look for another inflatable.
I always inflate my inflatables to the hardness of a basketball, and operate them that way. I'd suggest you do that when checking out an inflatable to buy. If the seller freaks out and won't let you do that, then walk away. Nothing ruins an inflatable , RIB or otherwise, quicker than not operating it fully inflated.