Hi, a sand base always works out better on the second liner. The sand has packed and just a little bit of smoothing really makes it flat, and it is more likely to stay that way.

As for reusing your existing liner it is kind of a crap shoot. You may get the bottom smoother but wind up with a lot of wrinkles becouse of shrinkage.

My rule of thumb would be any time after the second season do not even consider draining the pool. Just live with it until a liner change is needed.

Hope that helped some, Dennis