Sometimes adding one's personal insight and philosophy to a discussion can be good, and helpful. Other times, it can be seen as being on a soapbox and arrogant. At the risk of hoping to contribute in the category of the former, I'll step out on the limb to offer my opinions.
I really think that touching up and selling a print is very risky, if the print is going anywhere of any consequence. What is at stake is your reputation as a printer, artist, photographer, etc.
In woodworking, there is a phenomenon called "cold creep". When blemishes are filled with putty or glue/sawdust mixtures, etc., eventually the filled area works it's way up and through the finish, creating a raised area that becomes visible and can be felt by rubbing the fingers over the area. This can happen when gluing boards together, as well, in certain humidity conditions where the joint will become noticeable. This raising up, occurs because the material is hygroscopic, meaning susceptible to changes in relative humidity of the surroundings. Canvas is not unlike wood - we all know it loosens and tightens seasonally, just with changes in humidity. Like wood that changes hygroscopically, coming and going, expanding and contracting, canvas does the same thing and forces the material up and out, eventually creating a scab or scar. Of course this takes time, but if a canvas is moved from a dry climate to a wet climate, or vice versa, it can accelerate this process.
What is at the root of this phenomenon is the condition known as co-efficient of expansion and contraction of dissimilar materials. On a much larger scale, this relates to coefficients of thermal expansion of dissimilar materials. Of course we are dealing with a microscopic variant of this phenomenon, but it is very real and can in time, become a problem. Or not.
I think filling any area with a patch is risky, for the reasons stated above. Chances are likely that at some point, the "fix" will fail. So the fixing is like a ticking time machine.
Either you subscribe to the theory that these things can go wrong, or you don't. But in the case that it DOES go wrong, it comes back to reflect on you as a professional.
Here's the thing, basically, what I'm saying.
If you don't care about your customer and don't care if the fix breaks down, then all's good.
If you care about your customer, and you care about your reputation, then NOT GOOD.
A reprint is cheap when it comes to reputation insurance. Do you believe in yourself as a photographer/printer? Can you afford to take the risk that something might happen that could reflect poorly on you?
A case in point. Years ago, I fixed a piece and sold it. Many years later, the piece ended up in a museum and the fix had failed. It was visible. It had failed, visibly, and now it was in a museum, and beyond my control to repair or re-do. When it comes to museums, there are no re-do's. I was mortified and to this day, still am, because I was too cheap to do over, or thought I could just fix it and let it go.
Of course, if you think you'll never be in that situation, then I guess just fixing and letting it go, is good enough. Problem is, "just letting it go" puts the thing out of our control. It is gone, period. Never know what's going to happen to it once it leaves your hands and becomes the property of another.
It's when your piece which has failed just gets tossed, however, and the work of another photographer takes its place, that hurts too. Because the client will never come back.
Like I said, not meaning to be soap-box'y - just offering my perspective after 40 years of experience.
A new print is a CHEAP insurance policy.
FWIW / YMMV
