We did our companionway steps by scraping off what was left of the original varnish. While it's fine to just sand down intact varnish and add more layers on top, you need to get at the wood to fix any blackening or fading (with bleach and oxalic acid).
After varnish removal, a cabinet scraper will bring out the wood patterns very beautifully (optional step though).
Then just pick any varnish you like and build up new layers. The final coat should match your boats interior, which meant satin on our J&J Bavaria. We used some chemical product that looks okay but was exuding toxic smells for months, so wouldn't recommend. Next time I'll just go with time honoured Le Tonkinois again