I have some paintings with oil paint painted on top of an acrylic base on some parts but not all of the canvas. I want to put a separate gloss coating on to seal the surface before a coat of your GOLDEN MSA varnish. What do you recommend for the non-removable seal coating?
In a case like this, apply the MSA Varnish directly to the painting. You could have applied Soft Gel (gloss), thinned 2:1 with water, before you began with oils, but unless you can easily coat just the acrylic areas and not overcoat the oil paint areas, don't worry about it.

Some people will use an alkyd medium over the artwork to seal off the painting before vanishing, but it may be unnecessary unless there are areas of gesso alone or thin layers of acrylic over gesso. The varnish can absorb in to these sections and result in an uneven application.

If you are concerned this may happen, but do not want to apply the alkyd medium (Liquin, Galkyd, etc.) then apply Gloss MSA Varnish first, then the sheen of your choice. If the painting requires future conservation, top layers of varnish can be removed to clean and repair, and the absorbed varnish remains. With any of these applications, it would be helpful to note on the back of the stretcher bars (NOT on the actual canvas, as it may bleed through or cause a texture if glued on, etc.)