Thanks for your reply Connie! I am no expert, it is just a topic I have worried about and debated, and is a dilemma for me too.
I didn't imply Gamvar would yellow over time, there are numerous reasons a conservator would remove a varnish (an underlying problem in the paint, darkening of the painting, damage to the canvas, etc.....) and Gamvar would seem to be one that is easily removed, but I don't know if that would be the case if it had been applied immediately after painting, rather than waiting 6 mos.
And to answer your question, yes a painting can be cleaned without removing a varnish, this is the best possible situation. Surface dirt can be cleaned -- sometimes with just water or spit--- however there is no "picture cleaner". There are hundreds of solvent mixtures and gels used to clean dirt, varnish, synthetic or natural, etc.... A conservator is needed as they know how to test solvents, and what steps to take, what is safe to use... I have seen a lot of paintings that were damaged by old and or bad conservation. I have also seen many paintings that have no retail or market value --but that are very important to an owner-- that cannot be cleaned or repaired because there is a problem with removing a varnish.
Just a thought---Van Gogh did several of his paintings in one day, and many people thought they did not have much value.....(ditto for many Impressionists, any alla prima painting, etc). So who knows what the future value will be.
And I am absolutely not saying you are wrong to varnish when you do. I do not know --- it may turn out it is perfectly fine to use Gamvar in the first month of painting, but for me I would rather not risk it. A painting can always be varnished in the future. I am also curious about the addition of wax to the surface, and how that affects the painting.
I have also seen new paintings ( a year old or less) that have been completely destroyed by plastic wrap or bubble wrap, which was used for shipping, either melting onto the surface or leaving imprints. New paintings that appear dry are still delicate until completely cured.
I don't mean to be so long in my answers, it is just a complicated issue.