How to Seal a Stone Fireplace Hearth

A fireplace is not just a focal point of any home, it’s the heart of the living space. If you don’t seal your stone fireplace hearth, air and water will get in between the cracks and can cause major damage to your chimney. This post will give you some tips on how to seal different types of limestone slabs with masonry sealant!

A sealer application procedure varies based on the type of hearth you have, the construction of the fireplace, and the desired look you wish to achieve on the surface of the hearth. Let’s discuss the steps involved first to seal limestone hearth slab. It is not recommended to go DIY on this project and hire your local masonry repairs.  

Sealing Limestone Hearth Slab

  • If you’re applying a penetrating masonry sealer, then mix it as recommended on the label.
  • Pour or pump some of the mixture onto your floor covering so that you have enough room to work. Then, secure the limestone slab(s) in place with temporary supports such as wood planks and bricks.
  • If necessary, do any cutting beforehand to prepare for installing a cultured marble veneer using a diamond blade wet saw equipped with water, dust collecting shroud and side handle. Use extreme caution when doing this because they are very sharp! Afterwards, clean off any remaining residue from the cut face with a wet rag, followed by wiping dry with another clean rag. Place all pieces where they won’t get stained or scratched.
  • If you’re applying a sealant to natural limestone, you can use sandpaper with #180 grit (grit of sand particles on the surface of the stone which is less abrasive than #80, #60 and #40 grit sandpapers). Sand parts of your hearth slab where it’s unusually shiny since this indicates that the sealing material has worn away. Furthermore, cut yourself some sanding sponges into smaller pieces using scissors. These are more effective for sanding around tight areas like corners.
  • Apply an adequate amount of sealer to all sanded surfaces using a brush, roller applicator or back-and-forth motion with your garden sprayer. Then let the finished hearth slab dry for at least four hours. Once the sealer has dried, check to make sure no areas need touching up. If so, reseal them and let everything dry for another four hours before proceeding to the next step below.
  • Remove any masks or gloves and thoroughly clean off any residual sealant from your skin and work area with a wet rag, followed by wiping dry with another clean cloth.
  • Apply some type of protective coating such as an acrylic floor wax if you want to create a shine on top of the sealer. Otherwise, go straight to sealing cultured marble veneered a top natural limestone.

Applying masonry sealer to natural limestone

  • Sweeping, vacuuming or dust mopping the floor will help you identify areas that are still dusty. Remove any sanding residue left by step 5 above since it may contaminate the sealer and interfere with its adhesion (it’s also important to do this especially if you’re not applying wax).
  • Mix your sealant as described on the label, then pour it into a container large enough for your brush, roller applicator or garden sprayer. If using a roller, choose one with an 8″ nap cover (meaning thickness of material used) because thicker nap covers tend to clog up less frequently than thinner ones.
  • Use a 4½” brush or roller applicator to work the sealer into crevices and other recessed areas. Use a 9″ roller for flat expanses. Then wait 15-20 minutes for it to penetrate any remaining sanding residue left by step 5 above before starting to apply it with your garden sprayer , which does an excellent job of distributing the material evenly and thinly over larger areas (it works best if held at least 2 feet away from the hearth).
  • After waiting another 15-20 minutes, use a dry rag to wipe off any excess sealer that’s still visibly wet. Mop up the remaining puddles and let everything dry overnight (or longer if recommended on label) before continuing with step 9 below.

Applying sealer to cultured marble veneered atop natural limestone

  • Sweep or vacuum floors, then go over them lightly with a damp rag to remove any dirt and dust residue left by sweeping. Since cultured marble is delicate, don’t drag anything across it such as saws and other tools that can leave scratches and gouge its surface.
  • Mix enough sealant as directed on the label for your first coat , which should last from four to ten years depending upon how much foot traffic it gets (the more it’s walked on, the faster it will need to be redone).
  • Pour sealant into a clean container large enough for your roller or garden sprayer (if using a roller, use one with an 8″ nap cover), then roll over stone surfaces in back-and-forth motion with roller applicator. When applying with your garden sprayer, hold it at least 2 feet away from the hearth. If you’re not 100% satisfied with how smoothly and evenly this first coat has covered (it should completely conceal all of your stone’s imperfections), wait until next day before applying another coat.
  • Wipe off any excess sealant that’s still visibly wet after 15-20 minutes. Let everything dry overnight (or longer if recommended on label) before continuing with step 9 below.
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