Kreem threads are always fun. Lots of people hate the stuff, even if they have never used it. I have defended Kreem a few times in the forums in the past as I had good results with it on several tanks.
I installed a F2/F3 tank on my '75 750F back in 1990 after an abortive attempt to get a big dent out of the original. The replacement was from a donor bike out of a wrecking yard and had some rust in it. So I used a Kreem kit on it and it came out great. Again, this was in 1990, and the Kreem lining had held up for 18 years. Then about two months ago, I noticed some mottling inside the tank while filling up. It has since gotten progressively worse. As you can see in the pic below taken today, it is bubbling up and flaking off. It is doing this in the entire tank.
I see a couple of possibilities here. One is that 18 years is the lifespan of a Kreem lining. Another is that there is something new in the gas that is attacking the Kreem liner. I am leaning toward the second possibiliity. After the first of the year, most gas stations around here changed to gas labeled as having 10% ethanol. I am not inclined to think it is the ethanol alone that has done this since I have used gas with ethanol in years past when I couldn't avoid it and had no problems. The word has been that since they quit using MTBE there have been other new additives besides the ethanol to take its place.
At any rate, I can no longer stick up for Kreem. Because of all the raves here and other places, I used POR15 for the first time last summer in a friend's Concours tank to see how well it works. It will be interesting to see how the POR15 holds up.