I fiberglassed a crack on my sink in a 78 sovereign
on the back side and repainted with marine paint, painted shower and tub at same time. Cracked reappeared two years later and I epoxied on surface and repainted has held up good for three years! I'd do back side if possible, I replaced shower wall (closet wall) with marlite and had to remove tub and shower wall and it was a major project! Surface repair would be my choice unless that portion can be removed easily.