Removing wood glaze from stone tiles

Good day, I am asking on behalf of my father.

Last year we treated our wooden fence & pavilion in the outdoor area with colorless wood varnish/wood glaze. Since the seller said that the wood varnish could simply be removed with ?Turpentine/Acetone? (Multichimica Acquaragia Extra) (presumably only immediately after contact), we did not work particularly carefully and some paint spread onto the substrate on the stone tiles.

After a few hours the wood varnish had already set firmly and continuous scrubbing with the help of ?Turpentine? led to no improvement at all. In the following days we tried other agents and even a steam cleaner but saw no improvement.

The stone tiles are thinly coated rough on ~ceramic/clay.
I am not sure about the paint whether it was glaze or varnish. But I think it was a wood varnish ~acrylic varnish.
Edit: I am not sure if it was actually this paint but my father mentioned possibly: solvent-based protective glaze - Arreghini classic (hydrocarbons, C9-C12, n-alkanes, isoalkanes, cyclenes, aromatics. C9 hydrocarbons, aromatics. n-butyl acetate. Xylene - isomer mixture)

Is there any possibility/solvent to remove the firmly set paint residues (brown stains in the attachment)?

I guess it can be easily removed with Nitro thinner or acetone. But as always, please test beforehand on an invisible spot. You don't want it to damage the tile surface.

Nitro thinner is available at Sonderpreis Baumarkt or at Wreesmann and such cheap junk stores for 2 euros.