Clean historic masonry using the gentlest-means-first approach per NPS Preservation Brief 1, including required test panels

Steps

1. Retrieve NPS Preservation Brief 1 (Assessing Cleaning and Water-Repellent Treatments for Historic Masonry Buildings) from nps.gov and confirm the guiding principle: use the gentlest method that will effectively and safely clean the specific masonry, escalating in aggressiveness only as needed
2. Identify the soiling type (dirt/atmospheric grime, biological growth, paint, graffiti, staining) since the correct method depends heavily on what is actually being removed, not a one-size-fits-all treatment
3. Start with the lowest-impact methods first: low-pressure water washing with natural or mild synthetic-detergent bristle brushing, before considering any chemical or abrasive method
4. Always perform cleaning trials on inconspicuous, representative test panels and let them weather/dry fully before evaluating results and scaling up to the full building
5. Avoid sandblasting or other abrasive dry/wet blasting on historic brick, terra cotta, or soft stone per Brief 1's explicit caution, since abrasive methods permanently remove the harder fired outer surface and accelerate future deterioration
6. Select and pre-test any chemical cleaner (acidic or alkaline) for compatibility with the specific masonry type, and ensure thorough rinsing/neutralization to avoid leaving residues that cause efflorescence or staining