Hi Martin–First off I would like to say that those varnish videos that show people pouring bottles of varnish on a painting really make me cringe. I am of the conservative mindset that you only use enough of any material to accomplish a particular task. As to the tools of application–I have only ever used brushes to apply varnish like the ones seen here:
I am surprised to learn MJA uses spray varnish. Not that I have any problem with it whatsoever. Frankly, I have very little experience with it, but a number of the artists that I had worked with cultivated some serious love/hate relationships with it.
As to the liquin, I’ve never had to take any special steps to ensure successful application. I just applied it in a conservative manner (again, NOT like the pour-happy varnish porn you find on social media-LOL!)