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Please don't use sand! (Sand won't just stay in the divots, it will travel all under the liner and you'll be able to see and feel all the grains under the liner.) Speaking of which, about the best thing you can do, for any liner install is sweep the floor 3+ times before you drop the liner - a grain of sand beneath the the liner , will look like a 1/4" pebble - under the right lighting.

What I would suggest is a vermiculite mix for leveling the low spots. (If you need info on mixing and apply7ing verm, let me know and I'll do my best to describe the process. If you do go with the verm, pick up a 'wood float' (wooden smoothing tool) and a trowel (a 'pool trowel', with rounded ends works best, but you could get away with a regular rectangular trowel).

I'll let you respond before I belabour this more