Even if a varnish stain has gone unnoticed for years, it is relatively simple to lift from an untreated floor. Lacquer thinner will remove most small stains and spills. If the stain stands up to that, try chemical paint stripper. Designed to remove old paint and varnish, it will make short work of the stain.
Remove Varnish Stains with Lacquer Thinner
Saturate the stain with lacquer thinner. Allow it to sit for five minutes.
Scrub the stain with a natural bristle brush, then wipe it up with a rag. Repeat applying the lacquer thinner, scrubbing and wiping until the stain is removed or you make no more progress in lightening it.
Wipe the lacquer thinner away with a rag soaked with water.
Remove a Stubborn Varnish Stain with Paint Stripper
Pour a small amount of the paint stripper into a plastic container.
Paint a generous layer of the stripper onto the stain. Be sure to stipple it into the nooks and crannies of the concrete where the varnish has penetrated. Leave the stripper in place for the manufacturer-recommended amount of time -- usually 30 minutes to one hour.
Scrape the stripper and loosened varnish up with a plastic putty knife.
Soak a rag in the stripper. Use the rag to scrub up any residual varnish.
Soak a rag in water or the neutralizing solvent recommended by the stripper's manufacturer. Use it to wipe up any residual stripper.