Except burns or some two-component finishes that are too strong, there are few irreversible damages. To identify the nature of the applied product, scrub the surface with a rag soaked in white spirit. If the color is transferred to the fabric, the wood is waxed. Otherwise, it is likely varnished. Anyway, a good scrubbing with soda lye and brush is a prerequisite before refurbishment.
All of these products are harmful, so wear chemical-resistant gloves. Work in a ventilated area or outdoors (but not in direct sunlight).
Remove the wax
If the wood is waxed, apply a scourer with a cotton wick.
Renew the buffer regularly, until it does not stain anymore. Rinse with fine alcohol to varnish.
Spray on the varnish
For a varnished wood, use a special furniture stripper.
Shake the aerosol, then deposit a fairly thick and regular layer (hold the nozzle to about 20 cm).
Remove the varnish
Wait until the varnish layer is blistering.
With a blunt edge, scrape the raised layer (wipe the tool regularly with a cloth).
Work in the direction of the fibers.
Neutralize the stripper
Immediately neutralize the action of the stripper with a cotton wool pad soaked in water or fine alcohol to varnish, depending on the product used (see the instructions).
Be careful not to soak the wood.