I like to use a spray clear coat. There a few brands to choose from.
Waterlox is amazing. If you want something to look like you bought it in a furniture store....this is the stuff. I show you exactly what we did to get this amazing finish on a piece of furniture I re-upholstered. The pictures in the blog post aren't very good since I didn't know what I was doing with the camera. But if you follow the directions closely, you will get a great finish. Here is my blog post:
Wood likes oil but oil yellows. Doesn't really matter too much on stained wood since its a yellow base. We use clear furniture lacquer on our pieces.
Any good brand will work. Everyone has a personal preference. Thru the decades I have tried all kinds of brands and finally settled on Minwax products.
Well, actually it depends on what kind of wood you need to varnish. There is a lot of stuff for waterproof wood finish. It colored differently too. I can't tell you too much about it, but I can tell you where to search for this information. One of my friends sent me a link to one site with articles about this stuff not so long ago. I saw an article about varnishes and stuff there. You can look right here: pursuingoutdoors.com. There is plenty of information about how to finish wood work and etc.