What is stopping varnish/stopping putty?

I suspect you're right.  Several of the paint/varnish manufacturers (e.g., Minwax) make "pre-stains" or "wood prep;" I don't see how a coat of it could do any harm.  In addition to filling up the biggest of the wood pores, it's reputed to make the finish coat of varnish less "blotchy."  That may or may not be a problem that's worth worrying about, depending on the wood species.  Cherry, for instance, is often said to be prone to blotching, though I've used quite a bit of it and haven't had that particular difficulty with it.

Hmmm, what are they declaring the use for?  Stop-out varnish is used in painting and printmaking as a sort of "resist", so no colour will adhere to it...but I can't imagine why you would need it on a sailing ship kit.

          greg

Welcome to the Forum!

The English "instructions" in those continental European sailing ship kits are notoriously bad.  Without knowing the precise context I can't be sure, but I suspect "stopping putty" is wood filler and "stopping varnish" is just varnish.

My favorite brand of wood filler is Famowood, which is available from places like Lowe's and Home Depot.  When it comes to varnish I'm a fan of Minwax products - but there are several good varnish brands out there.