Cleaning & Sealing Moldy Limestone Patio (Shady, NW Iowa)

Question: Limestone block patio (south side, shade from maple) turning dark—mold? Can I whiten it?

Answer: Alkaline/Neutral Cleaner, Dilute Bleach, Then Impregnating Sealer

• Use commercial limestone cleaner (alkaline/neutral pH). Acid erodes limestone. No muriatic acid.
• Or 1:1 bleach:water, dwell 1–2 min, scrub, rinse well.
• Test cleaners/sealers on hidden spot 24 hrs.
• Rinse with gentle hose spray—no pressure washing.
• Apply high-quality impregnating limestone sealer (2 coats). Prevents mold return by sealing pores & keeping water out.

Mold thrives in damp, low-light stone pores. Sealer denies habitat.

—Mark Harshman | mahdrafting.com