If I spray Desert Varnish on a Canvas Paper inkjet print, will it be sealed so that I could paint over the inkjet print with Oil Paint?

I want to use my inkjet to print faint lines, which I'll paint over. Sort of like an underpaint. though I want the canvas to be prepped so that I can paint directly over it.

The Moab Desert Varnish is not really a sealant as it is a protective spray to prevent deterioration to the print from harmful UV. Using oil paints on top of a print protected by Desert Varnish will result in the oil paints bleeding through the paper. If you'd like to seal your canvas there are a variety of third-party sealants available from companies like Krylon, Liquitex and Lexjet. But we do not know enough how the oil paints will react to the sealant. You'll have to run some tests.

If you are just using the inkjet printer for fine lines to be used as guides, you don't have to worry about the oil paints interacting with the inks. We generally don't recommend using oil paints with printed images, as the solvents can create issues with the printed image.

If you are using aqueous (inkjet) coated canvas, that may be enough of a surface prep to hold the paints, similar to a gesso. I'd recommend testing to find out if it is sufficient. If not, a faint wash or clear gesso would probably be the best route.