Noah was told to cover the inside and outside of the Ark with pitch.
Genesis 6:14 gives the instruction immediately after identifying gopher wood as the ark’s material. The pitch was applied to both surfaces, creating a protective coating for the vessel.
The Hebrew wording is related to terms for covering or coating, and many English translations render it as pitch, bitumen, or a similar waterproofing substance. The central idea is a seal against water rather than a decorative finish.
This detail has a parallel in the story of Moses’ basket in Exodus 2:3, which was also coated with a waterproofing material. The two stories use similar language for protecting a container from water, although they describe different objects and events.