Experience & Opinions PLEASE!
1-I'm replacing 3' of Cast Iron ptrap, pipe and wye's with ABS.
2-my goal is to further preserve a sound, but 50 yr old CI shower drain set in 3" of concrete shower pan in a tiled shower.
3-after gentely cleaning interior of drain, I plan to 'paint' on several layers of 2 part fiberglass resin.

Anyone ever do as above? Can it work? Worst I can think of is f-glass flaking off. Beyond the 3', waste line becomes 4" ABS.
Many thanks, John M.