+1 Weldon.....available online....of local purchase.
Cracks are tough. Got to make sure it is clean and dry. I suggest Weldon 16.
Weldon 3 is very fluid and dries too fast....for experts.
Weldon 4 is also fluid but gives you more work time.
Weldon 16 is thicker and good for filling slight imperfections.
If the Weldon doesn't work, I'd suggest RTV silicone.