I am replacing the tile on a basement concrete shower basin that has been leaking into the main area of the bathroom. The basin appears to have been poured after the floor and there are gaps between the floor and the basin. In trying to seal the basin I've asked the pros at Home Depot and they said th install a PVC liner over the concrete and seal the corners as per the manufacturers directions. Reading the directions the liner is to be installed between layers of thinset. I was not going to use thinset, but a premixed mastic.

What do you recommend? Should I install the liner and the mastic , or something different?

Hunter, did you really say "asked the pros at Home Depot " The advice probably wasn't bad but never confuse HD with a pro. Don't buy the premixed mastic, it is likely modified thinset at best and you need thinset, not pre-mixed. Before we start rebuilding an entire shower pan lets take close look at the existing pan, weep holes and drain. Are there any cracks in the pan, can you get to the weep holes to insure they are not blocked? Back to you Hunter.

Yes, the "Pros" is what I meant to say.

The shower basin has a cast iron drain set in the concrete (original to the floor built in 1961) the "curbs" were poured on top of the floor. The curb separating the room floor and the shower floor is in good condition, the two end curbs were not poured correctly, not full height and they are gapped from the front curbing. I have build up the ends using quikcrete hydrolock concrete, filling in the gaps.

The actual floor is in good condition, mosiac tile was installed. I've removed the tile and cleaned the concrete floor in preparation of the new tile floor. I am concerned about water leakage between the shower and the rest of the bathroom. There was leakage before, causing a failure of the linoleum tile adhesive in the main area of bathroom.

How should I seal the shower basin? Liner or roll on sealer or??

Good, more information requested. Have posted more information.

OK I think I'm getting the picture. You never really had a proper shower pan to begin with. You had a floor and a curb added to it and that probably wasn't water tight after the first year. You will need to add a membrane, layer of cement, chicken wire andother layer of cement. I should not advise you further because I am not a fan of built in place shower pans. I take the chickens way out and buy Lasco redi-made fiberglass shower pans.

Tiles on the current base have lifted. What do I seal the concrete base with before I tile? Is there products to remove existing bsealers?

The liner goes under the concrete base - not under the thin set. You have to remove the layer of concrete, install PVC liner - as per Home Depot experts' directions, re-pour concrete... and then you install thin set and tile.