I have some further information, courtesy of MJA:-
To apply a final varnish using a brush is to ask for trouble; varnishing is a difficult craft that needs a trained craftsperson. This was once a part of a framer’s training, but is no more, and no framer today will varnish a painting using a brush, as the risk of damage (and thus, lawsuits) is so high. It is safe to use a brush on a small painting, but normal dimensions (and bigger) need to be painted in small sections, and these require a tricky system of overlap that does not noticeably build up material at the overlapping parts.
Interestingly he also recommends an intermediary light coat of liquin:-
Liquin (see Winsor & Newton’s website) never changes colour after the first 24 hours (it darkens ever so slightly as it dries overnight), it is flexible and it is permanent. With an isolating varnish, the painting is effectively “locked-up,” and subsequent varnish removals cannot hurt it.
He’s a big fan of liquin, and favours liquin original above all the other variants.