Is there a difference between a lacquer based spray and a solvent based spray?
This question is a little bit out of my realm of expertise, but traditionally lacquers and varnishes have resins suspended in a solvent other than water (water is a solvent, too), typically aromatic hydrocarbons. The value of acrylic resin emulsions which are water based, is that they tend to be less toxic and easier to clean the tools after application. Beyond that, most folks use lacquer and varnish rather interchangeably, but there are other technical distinctions for those that want to be sticklers for accuracy. For example, a traditional varnish is made up of a resin that is kept in suspension by the addition of thinners and also contains drying oils that will help dry down properties, whereas a lacquer is made up of resin/solvent mixtures that also have dissolved nitrocellulose in them. The film forming properties of acrylic emulsions tend to be poorer at creating coatings with the highest gas and vapor barrier properties compared to resins, even the same acrylic resin, when suspended directly in a solvent rather than emulsified. I'm sure others on this forum can add further nuances that account for various technical distinctions between acrylic emulsions, resin varnishes, and lacquers. Or google the topic. You will find out more that you ever wanted to know
The above is a good technical answer, the information I provide is more basic.
Most people are familiar with Shellac - this is a common form of lacquer which I recall thinking my father used to use on wood furniture. Apparently nail polish uses it too. http://en.wikipedia.org/wiki/Shellac
I suppose you could try Shellac on a canvas, but I have no idea what it would do or why it would be preferential.
Further, a solvent based varnish by my definition is a solution that uses mineral spirits or turpentine as a base for polymer and resin particles to be suspended in.
Through the curing process, the solvent evaporates (exposure to air) and the polymers bind with the resins to form a tight honeycomb surface which is both flexible and mar-able.
I know this, because I use this myself and have done so for over 5 years now.
"Epoxy" finishes that use resins also can be used. The effect is similar.
Mineral spirits mixed with oil based pigment have been used for centuries, we commonly call them oil paintings - more recent is the invention of polymers and the introduction of acrylic paintings which float polymers (plastic) in an aqueous base. In both cases, the acrylic painting or oil painting are then varnished with a clear solvent based varnish to protect the painting underneath. This is considered a truly archival method of painting because the varnish CAN be gently removed and reapplied if the surface layer is damaged (there is an isolation layer which separates the surface varnish from the paint layer). Removing the varnish does not affect the surface layer
Water based solutions innovated in the last decade or so for use on giclee prints perform a similar function, but cannot be removed - so archival terminology in this context is a bit of a misnomer. The print is really being coated with a permanent coating which bonds directly to the canvas, and it does enhance dmax, and also afford protection from fading, cracking and environmental pollution - but if the surface is damaged, repairing it is near impossible if not very very difficult. ( Its easier to just print another one and coat it)
Shellac is not flexible and while we want a coating that will protect our canvases from fading, cracking and the environment, it must not be so rigid it is inflexible to movement or the varnish layer would crack.
Nowadays, people do all kinds of neat things with paper, canvas and photography and there are no "rules" - however, the danger in experimentation is that what may look good today doesn't stand the test of time. What you put on top of your ink and paper may react and yellowing or flaking, or even cracking can happen in months.