Original question was about fiberglass resin extending the 50 yr life of cast iron shower drain. Got the idea from lining our clay sewer pipe. Since your advice on the 22nd, had a plumber over to trouble shoot; he would replace from p-trap to the 2" ABS, about 5'& sidestepped the fg idea. Today a large LA plumber co. visited and confirmed 2" is a small as they go; seems you lose 1/4 X 2 when you line pipe, netting a 1" shower drain. That won't do.
Will diys fg & to prevent wicking, will use 2 part polyurathane boat paint as final coat. Yes, I know it is an overboard solution.
Thanks for your enlightening input. John M.