I recently used an Interior Wood Varnish for the first time and was extremely impressed by the results. I used a colour tinted varnish from Valspar (available at B&Q stores) on interior wooden doors and used it to spruce up a wood framed mirror and fireplace surround.
These semi-transparent varnishes are available in 32 colours (bold colours of green, blue, red etc. as well as traditional woody brown colours), and by painting on thinly you get a wood stained effect with the wood grain patterns enhanced and showing through .
The doors I painted were the original Georgian doors from the house which had been stripped back to bare wood by the previous owner of the house. I definitely wanted to keep the doors but the yellowy pine colour was not matching with the more neutral/earthy tones in the rest of the building. I went with a grey tinted varnish to give the wood a vintage aged look.

Original Colour of bare wooden doors. With my grey/white walls and grey floors this really stood out – need to tone it down!
When applying the varnish it’s much better to do several thin layers than fewer thick layers. I made this mistake at first out of impatience and by the time I’d thicked it on the wood grain patterns where covered over in a solid layer of grey. Not what I wanted.
You don’t really need to wait for the full drying time stated (1 hour) to do additional layers, by the time I’d painted the whole door once I’d start again on the second layer as the varnish had dried enough. After three layers I stopped but you can do fewer or more layers depending on how strong you want the colour.

Doors are now the colour of aged Oak.
The results you can see enhance the wood grain pattern. The Before and After photos here are reasonably subtle because of the colour choice, if I had gone for blue or red the difference between would much more staggering!

Full length door, fits in much better with its surrounding now it’s an aged grey.
From a 1 litre tin I painted 4 doors, one fireplace surround and one mirror frame – so it goes quite a long way!
Here is before and after of the fireplace surround and mirror in the newly decorated room:
This process takes little skill to do. Some preparation by a light sanding and masking off areas not to be painted and you’re away. Being water-based makes the cleaning up process much nicer than with usual oil-based wood varnish, no nasty paint stripper for washing.
I’d encourage people to give it a try as it can make a big difference, going for a bolder colour will give a you quirky shot of colour into your home which will have your friends asking after it as they’ll want to copy!