Hello,

I have a two piece cast iron shower drain in a fiberglass pan.
After 25 years the weep holes rusted shut. The bottom half of the drain & pan is perfect so I've replaced the top half of the drain with a new cast piece. It's a perfect fit. I'm planning on building up a new pan inside of the old fiberglass pan. I'm wondering what adhesive to use to attach the under side of the rubber pan liner to the bottom half of the cast iron drain? Since my shower drain isn't PVC, pvc cement won't work, how about silicon calk?

That would work fine, provided both surfaces are clean.

The caulk simply keeps water from getting under the liner in the event of a clog and backup.