Been a decades since I for years often applied varnish on various boats... both interior and exterior.
In NY, during 1960's, I did multi layers of Valspar varnish over mahogany wood.
We'd take the wood down to bear; being well sanded smooth and bleached when/where necessary. Then we'd work it back up with coat after coat of Valspar varnish. We sanded between each coat with very fine sand paper. Ten  coats varnish was not unusual on exterior. Interior was usually held to four coats. End result was mirror finish.
We found by many careful varnish coat applications the interior would last for many years without need for a mild sand and single refresher varnish coat. Exterior would go one and sometimes two years before the mild sand and single coat refresher.
I love the look of well varnished fine wood... who wouldn't!! However... being that I am no longer paid to varnish my boat... I now own a boat with NO exterior varnish and practically no exterior painted wood.
Worth reading: https://www.thewoodwhisperer.com/art...gular-varnish/
"Simple is as simple does!"