I have a metal door that was galvanized metal, and I wanted to coat it. I purchased from Home Depot a pint of some varnish clear liquid that looked cloudy in the can but rolled on clear, and I applied that to one door. It was $0 and claimed it would not yellow and was designed for out door use. I wish I remembered the name, but within 2 years it yellowed, and then it cracked and I was able to pull large sheets of it off the door. I redid part of it with Krylon clear, and it has not had an issue in 4 years. No crack, yellowing and very nice really. The doors get a beating from the sun regularly.
So one of those 2 canvas pliers, and maybe the Premier art shield, or Krylon.... Or see if I can take it to a auto body shop. They do a lot of matte finishes these days :-)
There are several important things to consider when it comes to finishes. Your statement about car finishes, not sure you were kidding or not. I have a friend who is a well known painter/sculptor who uses auto body paint and does several lacquer top coats over the finished work, mostly on aluminum. Consider that the American sculptor David Smith used car paint and car varnish on his sculptures that were outdoors for years in the 60's before museums bought them all and brought them indoors. The colors seem to be holding quite well.
I don't know what the Smithsonian would say about it, and I just haven't bothered to ask, even though I know a few conservators there.
The issue I would caution against is the aspect of the coefficient of expansion and contraction of dissimilar materials. This is always a problem and is at the root cause of all surface coatings - either success or failure. They ALL fail - it's just a matter of time. Canvas, not unlike wood is hygroscopic and absorbs the relative humidity of its environment, particularly as seasons change. The finished print is a "sandwich" if you will, of the actual substrate, the coatings for printing on, and the ink itself. Add to this a protective coating and specific things can go right or wrong. The automobile coatings are meant to go over paints that share similar characteristics, and are meant to withstand not only severe light but abrasion. It is a hard coat. Finishes such as glamour gloss and Timeless, etc., are "soft finishes" which first dive deep and sink in, then gradually buildup surface medium hard coats, which are still very soft in comparison to the auto lacquers.
Testing is the key to all of it. Which finish will wok for you that has the particular characteristics that will enable you to work with it to attain that sweet spot you're looking for.
It is doable, but testing and careful record keeping are required in your own "home lab", particularly as just in darkroom work, temperature is always a big factor in success or failure.
I hesitate to give advice on formulas, because I won't give information out on our own, but I can say that you can experiment with something like Timeless or Glamour Gloss, known standards, and begin doing either spray testing or roll application testing to begin to understand what is at stake in terms of what you can achieve, creating your own blend of finish that meets your eye.
My best advice is to do just that, learn what you can achieve on your own that suits you, then you can either go into production, or communicate this information to someone doing the work for you. It's all a rabbit hole, my friend, one brand makes you smaller, one brand makes you larger and one does nothing at all. Anything that is any good is a relatively closely guarded secret. And when you go through what you will to get to your goal, no way will you just give your formula out either.
Your eye is the arbiter of taste when it comes to this finish kingdom....
