Which Coating To Use On The I.D. Of Cement Pipe?

I need to know which coating to use for coating the I.D. of some large diameter (7') cement piping that has been in service for over 20 years. We want the pipe to last for another 20 + years. The pipe handles a high volume of water (110°F max)for a cooling system. Some of the coatings that I've thought about are coal tar epoxy, vinyl ester laminate, glass-flake reinforced coating. I'm concerned that the high volume rate of the water may remove the coal tar epoxy off the ID of the pipe.

I also plan to have the I.D. lightly sand blasted to remove residue, and then dried before coating.

1. If there are divets in the ID what kind of filler
should I use to fill in the divets?
2. Any comments about what type of coating I should use?
3. Where can I find information that will tell me how
many mils of paint will be removed per year in a
particular medium?

Thanks for your input!