Lowes offers Valspar Fast-Acting 128-oz. Deck Cleaner for $12.48 (Reg $15). Free shipping on orders over $35. 

Product Description:
Valspar Fast-Acting Deck Cleaner cleans and refreshes exterior surfaces by removing dirt, mold and mildew stains in just 10 minutes. This ready-to-use cleaner works on most exterior surfaces. Use before staining for truer color and more lasting protection.

• Removes dirt, mold and mildew stains
• Fast-acting cleaner; wash off after 10 minutes
• Ready to use - no mixing required
• Use before applying stains or sealers
• Cleans and refreshes exterior surfaces including wood decks, composite decks, concrete patios and pavers
• For use on wood decks, siding, fences, railings, steps and most exterior surfaces including concrete, brick, tile, stucco and composite decks
• Coverage - approximately 250-300 square feet per gallon