- 1). Sweep or vacuum the entire surface of the flagstone deck, including grout, to remove all dust and loose debris. Allow the deck to full air dry before moving forward with sealing. Ensure that rain is not expected for at least 24 hours.
- 2). Pretest the absorption rate of sealant on a piece of extra flagstone or in a test area of the deck. Conduct the test on a small area with your sealant product of choice. Penetrating sealants only need to be absorbed to a depth of an eighth-inch to be effective. By contrast, coating sealants rest on the stone surface and tend to create glossier finishes after application.
- 3). Insert a straw into the sealant and cap the top of the straw with your index finger. Lift the sealant-filled straw over the flagstone test area and drip sealant onto a central area. Place another drop of sealant onto the grouted edges. Time how long it takes for the sealant to spread and sink into the pores of the stone. If the sealant remains in a bead and doesn't soak in after 15 minutes, the flagstone may be non-absorbent or pre-sealed by the manufacturer or supplier.
- 4). Use the time results as a guidie for how much sealant to spread over the flagstones on the deck. Highly absorbent flagstone soaks in sealant within a couple minutes, while sealant tends to pool atop tight-pored flagstone. Don't put too much sealant on the flagstones if they tested as slowly absorbent. It's difficult to remove sealant that's been spread.
- 5). Pour the sealant into a paint tray and use a fine-textured roller to apply. Alternatively, use a wide polyurethane brush to coat all parts of the deck.
- 6). Start sealing the flagstone at a distant corner of the deck. To avoid having to walk across a wet deck, start at a deck side opposite your exit location. Place tools or items you may need behind you on the non-sealed portions of the deck. Move these items backward toward the exit so you can depart the work site without touching the freshly sealed flagstones for at least 24 hours.
- 7). Layer just enough sealant onto the flagstone to create a thin, wet surface based on your test plot results. If a portion of stone absorbs the sealant more quickly than another area, don't add more sealant to the drier area. It's better for a single layer of sealant to become absorbed to a depth of an eighth-inch over 24 hours than applying multiple layers that don't fully dry between applications.
- 8). Allow the flagstone deck to dry for at least 24 hours or other minimum period as prescribed on the sealant product label.
- 9). Repeat with additional layers as needed to create the finish you want on the flagstone. The porous flagstone will not become highly glossy like polished granite or marble, even with several sealant coats.
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