Black Iron Oxide vs Gloss Varnish

How close are they?

ΔE 77.7, poor: a clearly different colour, so not a substitute. Gloss Varnish is noticeably lighter than Black Iron Oxide.

Can I use Gloss Varnish instead of Black Iron Oxide?

Not as a straight swap; see the closer alternatives below. Poor match (ΔE 78). Gloss Varnish is noticeably lighter than Black Iron Oxide.

Can I use Black Iron Oxide instead of Gloss Varnish?

The same difference applies the other way round: Black Iron Oxide is noticeably darker.

ΔE is the colour difference between our two swatch estimates: under 2 is a perfect match (you’d struggle to tell them apart), under 6 very close, under 10 close, under 15 fair, under 20 loose, under 30 very loose, and 30 or more a poor match. Real paints also differ in finish, coverage and how they thin, so test before you commit.