I just finished a piece with quite a bit of rough impasto passages and was looking for advice on how best to clean the surface before varnishing. Also, is it easier to spray on the varnish or brush it on.
How long have you let it dry? and are you speaking about a final varnish coat ?
With the exception of a couple of touch up spots most of the painting and certainly all of the thick impasto parts have been finished for over a year. This will be the first coat of varnish it gets. Is there a great need for more than one coat?
To clean it of dust or that type of thing, I would use a piece of cloth dampened with turpentine and gently go over it and wipe it with a dry piece of cloth.
Let that dry a day or so. For varnishing it's your choice on spray or direct application. I use a small piece of cloth for that. Along with protection, you are also trying to even out the gloss or lack of gloss. I mix half gloss varnish with matt varnish to make a semi gloss. I don't like it flat or too glossy. The problem with spray cans is usually it's gloss only, I have never seen semi-gloss in a spray can. But you have to decide what kind if finish look you like.
When I do my half matt / half gloss mixture I usually thin it a bit with turp and usually give it two coats so I get an even semi gloss look.
If there was a semi-gloss in a spray can, I might use that myself. By mixing my own, I can control the gloss.