mixing wines....

Many of the really good store bought wines are mixed - like 80% merlot, 20%cabernet franc, etc.

What is the science behind doing this? I know what I like, and I do not always like the batches of wine that I make. But perhaps combining them would make a difference.

I've seen them do it at wineries, they sit around a table with beakers of wine and mix until they like what they have. Of course with just 6 gallons to play with, there is not a lot of room for error!

Has anyone mixed wines and can explain the procedure they use?

Wineries mix wines based on decades of experience, very little science involved. Most wines, even ones claiming to be a varietal, can have up to 30% (CA) other grapes. What you saw is exactly what they do for real. Once they have a good blend, they'll do it on a larger scale. Some of the best wines I've have been table wines that do not contain enough of a single variety to be called a varietal.

There are over 2000 compounds that can be added to wine without having to disclose them.

Dave's right. It really is that simple. I've watched in awe as our winemaker took out 4 bottles of different single grape wines, various sized graduated cylinders, a wine glass, and a spit bucket. He then went to town blending. Now, of course he knows all the varietals, their characteristics, and what he is aiming for so he knows which will be the base wine, and then what the other three will do and about how much to add.

At our winery they pay a consultant to come in and help. Blending is a highly skilled job, it takes years of experience to pick up the nuances that will produce a good wine. Sometimes it involves different batches within the same variety, other times they know roughly the varieties to be used (eg bordeaux blend) but have to decide the exact quantities.
I do my blending in primary, basically just chuck it all in and hope for the best.

As far as blending home wines, Terry Garey's book "The Joy of Home Winemaking" has a little advice. Not really specific instructions, just general guidance mostly sprinkled throughout the book. The fact that she actually created a blend called potato-mint should tell you she's not afraid to experiment a bit...