I discovered Insta-Seal does the same job as K-Seal, but Insta-Seal is cheaper to buy. Yasin from Bolton (Testimonial)

Unlike Liquid Copper or Metallic Seal-Up
INSTA-SEAL mixes with all Anti-Freeze!

Insta-Seal Permanent Block & Radiator Seal is compatible with all types of glycol-based coolants.

Formulated with ceramic micro-fibres and copper fines of varying size to
ensure a fast, permanent seal of all holes or cracks in the cooling system. The ceramic micro-fibres and copper fines flow into cracks that are too small to be seen by the human eye to provide a permanent seal. The micro-size
ceramic fibres and copper fill the entire crack from top to bottom to form a complete ‘weld’ within large and small cracks.

The copper fines also conduct heat across the crack or hole, eliminating hot-spots that normally form around the edges of a hole or crack.
Expands and contracts with temperature changes and is designed to be used in all types of metal engines and radiators — such as cast iron, steel, aluminium, copper, and brass — and radiators with plastic tanks.

Insta-Seal which is an advanced version of metallic seal-up but  does not require draining anti-freeze as required by metallic seal-up! Same product but outsells metallic seal-up 3 to 1!
I highly recommend this product.