I have installed a preslope for my pvc liner pan. When I drain it, there are two areas near the drain that a puddling about 1/8" deep. Unfortunately, the durock has already been placed on the walls with corner beads, etc and I can not peel back the shower liner without a lot of extra work. Is there another way to fill in the low spots with mortar that is effective, like cutting my liner and repatching it. Can I just redgard the next mud deck layer instead?