Hi Cindy--
I recently dealt with all of these same questions myself... For future, you can instruct your clients to have the framer leave off the backing paper (or ask the framer directly if you've got a relationship with them) so that the painting is easy to remove 6 months later. (You may also want to check at the same time that your framer is using rabbet 'frame sealing' tape along the rabbet of the frame to prevent acid migration from the frame to the canvas....) The framer may be willing to just apply the backing for you 6 months later once the painting's varnished... Or you can get
acid free frame backing paper from Dick Blick (and double sided tape) and put it on yourself. It's also good to put some vents or a cut-out in the back so that the painting can breathe.
If you're trying the GamVar for the first time, it's pretty straight forward to make... and the mixture with the 2 teaspoons of beeswax that I was so stressed out about earlier in this thread, I did wind up testing and using on a couple of my favorite paintings and they turned out beautifully. I've been offering clients a choice of finish... Glossy (straight GamVar), 'Satin' (the GamVar/2tsp Beeswax mix), and 'Matte' (which is actually the Golden's 'Satin' spray varnish but sure looks matte to me!) I've made up little sample cards of each finish on 5x7" gessobords and let the clients choose from there. So far every client has chosen the GamVar/beeswax mix.
The GamVar seems much more forgiving than other varnishes I've worked with... so I no longer stress out as much about applying the varnish itself, just dust/vacuum real well first and keep the cats out of the room while it's drying to prevent unintended little additions to the finish!.
Good luck!