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I'm a little late to this ball game, but wanted to toss in another angle. No one has mentioned the ground, which is where I've had tackiness problems.
If you buy a pre-gessoed canvas and you get the end of the roll, sometimes you can get a pooling of oiliness. It almost looks like big blotches of linseed oil have dripped on the canvas. It is impossible to see these oily areas until you have started the painting. They are visible because the paint dries extremely slowly and in a semi-oval pattern. (The first time I encountered this, It was on a Governor's portrait...not where you want to get a trial by fire like this one...).
The paint took over a year to dry into these areas, and dried in a tree-ring pattern, working from the outside in.
Now that I have spotted this defect, I've been able to see it in others canvases. You can "fix" it when you recognize it by rubbing the surface with mineral spirits before you start painting, removing all the excess dried linseed oil from the surface.
Unfortunately, if you have already finished the painting, it cannot be fixed. I ended up scraping the Governors portrait, and starting again. (...and yes, it had already been delivered...)
BTW, I've been using a variety of retouch varnishes over the past 22 years, and have never had a problem with tackiness that you describe. The primary problems with retouch, is if you apply it on fresh paint (paint less than a day old), the retouch varnish can cause your painting to dissolve and run down the canvas. If you use too much retouch varnish too often on the canvas, the surface become hard and brittle. I've never had a problem with tackiness that could be attributed to retouch varnish.
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