I'm building a shower pan the traditional way using a pre-slope, then waterproof membrane, then a mortar bed using sand topping mix. I laid my mortar bed and curb and it has dried but I've noticed that I neglected to create a level surface along the back wall (about 4 feet), and it has a slight dip in the middle, about .75" or so. My two options as I see it are, 1. add more sand topping mortar to level it out, or 2. use thinset to do the same. My questions are, 1. is adding mortar going to work? is there a problem with it cracking or not sticking to cured mortar? 2. if I use thinset to level it out, do I need to set tile on that immediately or can I let it dry and add more thinset when its time to set tile later on down the road?
thanks in advance