Remove any old cement from the seams by lightly tapping the back of a chisel handle with a hammer to chip away the material with the chisel blade.
Brush any last, stuck-on bits of cement away with a wire brush.
Eradicate any rust from the areas to be sealed, using the wire brush with white vinegar.
Clean the joint areas by wiping them down with a sponge dampened with a solution of hot water and dish soap. Rinse the sponge with fresh water and wipe down the areas again to remove the soap residue.
Wet the areas where cement will be applied before applying the cement. Apply a thin covering of cement along all joints, using a trowel or putty knife. Force the cement into joints to ensure they are filled. Stove cement caulk tubes are also available for extruding consistent beads, which can then be smoothed down. Re-wet areas if they dry before cement is applied.
Apply a thicker, second covering of cement along the joints, using the trowel or putty knife.
Clean up smears or drips promptly, using a damp sponge.
Start a mild fire in the stove and slowly build it to higher temperatures to heat-cure the cement.