Not paint thinner, Use paint remover.
you can get several types. Thin like water, thick like a gel. Mild or heavy duty. All should be applied while wearing gloves and in a very well ventilated area. Also wear safety glasses. One little drop in your eye is Baaaaaaad. and super easy to accidentaly do if you are using the brush.
The paint will bubble and can be scraped off with a plastic putty knife, or metal. But metal has a better chance of gouging the soft wet wood. Then use a stiff brush to get the remainder of the paint out of the grooves or deep grain of the wood.
the color of the bare wood is determined by the wood type. Pine is almost white, cherry is reddish, oak is darker, walnut is even darker. The remover will take off everything down to the bare wood.
do not use an electric sander. It will heat the paint or varnish and fuse it into the wood. Then it is very hard or even impossible to remove completely.
Mikeeee