What is the correct product to fill this joint with? If I should use a cement based product I do have a can of water plug which I'd rather use than buy a 60 pound bag of something else.
I use Quad Window Door and siding sealant.
You can get in grey.
Mask off the sill so you you do not get it where you do not want it. Fill the gap then take you finger and dip it denatured alcohol and use your finger to smooth it out, do not work it to much. Pull your tape and your done.
It will help to do a practice run on some scrap wood or something so you understand how to work with it.