While doing all this I noticed one of the older, larger pieces had some areas where the gloss of the paint was uneven, which left me dissatisfied. Although I don't always do this, in this case I decided the cure was to (drum roll!) varnish the painting.
I'm not sure why, but I always get a little nervous about varnishing. I think it's mostly from what I read about doing it as a young art student, primarily about the need for a "dust-free environment" - like that actually exists.
My practical experience has generally been relatively trouble-free. I use Liquitex Soluvar gloss varnish, thinned with just a small amount of turpentine, and paint it on with a 3-inch white bristle brush. I have less of a problem with dust than I do stray hairs off the brush.
So after pondering the notion for most of the day, I got home, cleared off a flat horizontal surface, poured out a little varnish and turps, and started brushing. After I finished I had enough leftover varnish to coat another older (but smaller) painting I'd decided some time ago to varnish next time I got around to it.
Clean up is relatively simple - use turps or mineral spirits, although in my opinion, turpentine cleans it out better. Important note! If you are thinning the varnish, read the instructions - it probably tells you to thin with turpentine. DO NOT MAKE THE MISTAKE OF IGNORING THIS ADVICE! The results otherwise can be awful - at least they were for me, years ago. Your mileage may vary.
So, why varnish? The usual reason given is to protect the painting, put a removable/renewable layer on the painting - all good advice.
Why do *I*?
Primarily because I tend to work thin, sometimes without a lot of medium, and the paint - especially in dark areas, can have a chalky, flat quality which looks sort of crappy. A layer of varnish will (for me anyway) make the colors instantly deeper, richer and uniform in their gloss.
It's a personal thing, to be sure, but over the last couple of years, I've pulled out several older pieces that I've not been completely satisfied with and hit them with a coat of Soluvar and WHOOSH - it's like a new painting. A good painting.
So, feeling brave? Just do it.
Don't use spray varnish (ugh); get a brush and smear that shit around.
You can thank me later.