How to remove a polyurethane stain on my sandstone step?

Q. What should I use to get out polyurethane stain on my sandstone step?

A. The methods for removing polyurethane from stone will vary, depending on what type of polyurethane caused the stain. Each type behaves differently once it hardens or seeps into porous materials like sandstone. 

Before you start cleaning, these clues may help you identify which type of polyurethane caused the stain:

• Oil-based polyurethane is amber-colored, slower to dry, and often leaves a glossy finish. Hopefully your stain isn't caused by oil-based polyurethane, because it penetrates porous materials more deeply, making removal tougher.

• Water-based polyurethane is clear, dries quickly, and is easier to clean up, especially if the stain is fresh.

• Industrial polyurethane coatings cure chemically. They are designed to be very durable. An industrial polyurethane spill will have a plastic-like appearance.

Because sandstone is soft and porous, it would be best to start with the gentlest stain removal method first and test in a small, hidden area.

For water-based polyurethane stains, if the spill is fresh, blot it with warm, soapy water using a mild dish detergent. Gently scrub with a soft-bristle nylon brush, then rinse thoroughly with clean water and blot dry.

For oil-based polyurethane stains, allow any residue on the surface to fully dry. Apply a small amount of mineral spirits to a soft cloth and gently dab the stained area. Avoid over-saturating the stone, because too much solvent can cause deeper staining. Switch out used cloths for new ones, as needed. Acetone, available at hardware or paint stores, may be used if mineral spirits do not produce desirable results. NOTE: Do not mix chemicals. After most of the stain is cleaned a film may remain. Use a soft brush to loosen the film, then rinse with warm water and dry. 

Industrial polyurethane coatings are very tough to remove. Use a commercial polyurethane remover or paint stripper that is specifically labeled as safe for natural stone. There are biodegradable, low-odor, or solvent-free formulas on the market. Always test first. Use extreme caution, as many strippers can etch or dull sandstone. Follow the product instructions closely, and neutralize the surface afterward with water and a mild detergent. 

If the stain has penetrated deeply, changed the color of the sandstone, or covers a large area or your stain removal methods have caused further discoloration or etching, it would be best to consult a professional stone restoration contractor. They have access to specialized poultices, professional chemicals, and polishing tools and the experience to understand what would be the best approach in your situation. To find a vetted PRO in your area, visit www.surfacecarepros.com and click on Find a PRO. They may be able to lift the stain completely or blend the stain with the surrounding stone without damaging the stone’s surface.

ABOUT THE AUTHOR Fred Hueston is the Chief Technical Director at SurfaceCarePROS.com and Director at StoneForensics.com. He is also the author of Stone and Tile Restoration: The Manual, a comprehensive online manual for stone and tile restoration contractors. [Learn more about Fred.]