Acrylic isolation coat and varnish query

There are a couple of schools of thought on varnishing acrylic paintings. Until I read some compelling research to the contrary, I subscribe to the two-varnish system where a sacrificial layer of water borne acrylic dispersion medium is applied and allowed to dry before adding a solvent borne picture varnish. Golden Artist colors has described this well. You can read about this here.   
https://www.goldenpaints.com/technicalinfo_varnapp1
If spraying is necessary and you do not want to purchase an airbrush, sprayer unit, or even a cheap compressed air can system, I would think that it would be reasonable to spray the painting with a coat of  B-72 as an isolation layer before spraying it with an appropriate picture varnish.
The availability of B-72 in a spray can is discussed on this thread about sizes.
https://www.artcons.udel.edu/mitra/forums/question?QID=200
Having written the above, acrylic dispersion paints are not my specialty. We have other moderators here who are far more knowledgeable about this subject. I will defer to their opinions if they differ from what I just wrote.

I should add a bit explaining my rationale of using an acrylic resin as an isolating layer under another acrylic varnish. The B-72 is only soluble in quite polar solvents like acetone/ethanol and highly aromatic mixtures. You should be able to remove an MSA varnish from a coating of B-72 using low aromatic mineral spirits. As usual all information about your layering should be recorded somewhere on the painting so that future conservators immediately know what materials were used on the artwork.

I am hoping that others would weigh in on this as well. I think that you would want to give the painting a few applications of the B-72 (allowing it to dry between coats) to build up the surface to that of a varnish and not just a super thin, fixative layer.

​Hi –
Just to be clear about one fundamental point, while it is true that we are big advocates of nonremovable isolation coats prior to varnishing, especially to address the concerns of  future conservators about their ability to remove a varnish from an acrylic painting without damaging the paint layers, they are not strictly speaking required. In fact most artists we know or work with tend to skip them and varnish their work directly. The reasoning here is that the isolation coat is non-removable and if anything goes wrong – from a fly falling into the material, to brush strokes or foam appearing –  there is no way to remove or repair those. So prior to ever applying an isolation coat onto any artwork of value, we would strongly recommend practicing on either test panels or older paintings to gain a high degree of confidence.
In terms of spraying on an isolation coat, you could try using something simple, like a Preval Sprayer, which we have in the States and is very inexpensive. It is often sold in auto part stores as it is used for touch ups in auto painting, but we see them also in the large stores catering to house paints. While the atomization is not perfect, and you will want to again practice to make sure you are getting results you like, it is a very low entry point for spray applications. If going this route, try out sprayable isolation coat recipe of 2 parts GAC 500 to 1 part High Flow Medium (earlier names Airbrush Transparent Extender). 
Beyond that, Brian’s thoughts on using B72 would be a viable route and worth looking into. No other sprayable options comes to mind, so would encourage you to look at a Preval-type sprayer, make the investment in an airbrush or spray-gun (especially if varnishing on a regular basis, these provide maximum control), or finally even look at skipping the isolation coat altogether and varnish directly. On this later, just realize  this will complicate any repair or cleaning by future conservators, so make note of what you have done on the back, regardless of the options you use.
On the use of fixatives, we do not feel they would be useful as an isolation coat or interleafing layer in an artwork. They are, first of all, designed to lay down a very weak and usually non-continuous layer that works to just barely bind loose particles to paper without encasing them in a resin layer. That is just a very different function that creating a true layer that can act as a barrier for a varnish, or prior to other layers in a painting  It can also be hard to find out what is specifically in a fixative, which can complicate knowing if the material is fully compatible with everything you want to do. As for OPEN, we would recommend using a light touch and brushing on a layer of GAC 500 if wanting to continue with other layers without disturbing what you have. You can read that recommendation here:

https://www.goldenpaints.com/technicalinfo_open#rewetting

But as we state, doing this will also slow down the curing process and lengthen the time when all the layers have fully coalesced. If you are consistently getting more open time than you need, think about blending the OPEN acrylics with a faster drying regular acrylic paint of medium to get working properties that match your needs.
Hope that helps!