Hi
Yes you fill the gabs and sand the goob down, also a very very
important issue --- you make sure the goob only attach to the sides of
the gab not the bottom, as if it stick to the bottom it can not move
and then as you can se everywhere where this foult occour , the goop
will open at the sides.
Usealy you fill in a thin strip ,some even use a flat string push it
down to cover the bottom of the seam and then fill with goop ------
emagine it attached to only the sides then the mastic can follow any
swelling where attached to 3 sides it will eventually fail at one of
the sides causing a leak.