I've started on some extensive varnishing around the house, namely the two built-ins, and the bookcase (which is in the garage to help minimize the fumes).
I generally use Satin Varathane for most of my finishing projects, and I thin it for the first coat (as suggested) 10%, but I either use mineral spirits (aka paint thinner), or turpentine. I generally prefer the smell of the turpentine, but MAN, the fumes are strong this time.
I had been torn with whether to stain the Fir on the built-ins, use a tinted varnish, or just use clear (plain). I ended up just going with plain varnish and I'm SO GLAD I did.
The Office built-in in particular turned out a beautiful shade of orange, and I'm actually surprised with how dark the final colour will be. It's going to look great, but like I said: I'm really glad I didn't add stain. Hopefully the orange tone will match the refinished birch floors when I eventually do those.
The built-in in the upstairs hallway is a bit lighter, since it gets very little sunlight, but it looks pretty good too.
I still have a lot of varnishing left to do. I only did the bottoms on the bookcase, since I need to do those first before I can flip them and do the rest, so I still have the bulk of the two cases to do the first coat, then all the doors, shelves, and back boards (I'm doing just the cases for now, and when those are done, I'll do the shelves and back).