You used to have to wait 24 hours after caulking a tub before you could use it…which means it never got done! This caulk dries in 2 hours! The preparation is the important part. Scrap out all the old caulk, wipe with rubbing alcohol and a cloth towel, then dry with paper towels. LET EVERYTHING DRY. Mask each side of the joint with blue painter's tape. Apply new caulk and finagle with your finger dipped in paint thinner. Remove the tape carefully and let dry. This caulk will resist mold for a long time. Click on image for link.
Or, use this tool and no fingers!