I want to try and varnish some mezzotints ( printed with oil based etching inks) to give them a high gloss smooth surface like a photograph. I was thinking of either silk-screening or airbrushing thinned down MSA gloss varnish. My question is regarding the need or not of an 'isolation coat' to help create a photo like finish. Your Web site only mentions using a layer of soft gel for this but being acrylic based wouldn't this be incompatible with the oil based mezzotint? The print is covered completely with a layer of ink but varies in its density in places and is printed on soft paper.
I think the MSA Gloss is an excellent choice for this application, and with testing either screening or spraying could achieve the look. I would think spraying will yield the desired results with less of a learning curve than screening, but depending on available application knowledge, I leave the choice up to you. You may also be able to spray light coats initially, and then brush apply heavier coats. The inks will be less likely to bleed if the first coats are not applied in a manner, which there is brushing, etc., directly on them.

You can thin the product up to equal parts with solvent and I recommend applying multiple layers instead of singular heavy coats.

You are also correct in assuming the gel isolation coat isn't a good choice for this. It may work if the inks are very thin and there is a lot of actual film absorbed into the paper, and you may wish to test adhesion as it would mean less exposure to solvent. Basically if the isolation coat works, you can just use Polymer Varnish. Testing all of these ideas is key to your success.