How can the seal not leak?
Take a picture of the sink. If the sink is relatively firm, fixed and does not require displacement.
Suggest that you use cf-2000 high efficient compound waterproof agent in flange joints, than the seal rubber board, economical, and good waterproof, and will not rot. Better durability.
Flange connection, plus rubber pad is enough. The pipe is like this, you sink this, I don't know.
