How to paint untreated indoor wood furnitures?

Hi there, I think you are saying the wood isn't painted or treated (bare timber)? First off would be to give the door a good sand down, starting with 120grit and then 180grit sandpaper. Clean the door down with meths or turps (not with sugar soap as the water raises the grain). Don't apply a primer - unless you want to paint the door! If you are looking for a subtle sheen finish then Danish oil is the thing to go for - it soaks into the wood and both protects it and gives it a rich lustre. Follow the instructions and you'll want to do 3 or 4 coats (but each coat is pretty quick) Good luck! The more coats of Danish Oil you do, the higher the sheen level. Once the wood is saturated, the subsequent coats build up on top, rather than sinking in and will leave more of a shine, especially if you don't wipe off the excess (although be careful as you can get patchiness - better to do several thin coats than try and get 1 thick coat to be even). Orherwise you would have to check with the manufacturer to see if a wax could be applied over the oil finish without having to strip it first.