Position an extension ladder beneath the fascia that requires sealing. Make sure the ladder is on even ground and that it is sufficiently tall to provide easy access to the eaves.
Put on rubber gloves before working with the sealing compounds since they can cause irritation to skin. Wear gloves made of thick rubber, such as those worn to clean with caustic agents.
Measure out a quarter-sized dollop of wood patch, which is a putty-like substance used to fill in woodworking. Roll it between the palms of your hands to soften it and make it malleable. Roll it into a snake-like shape measuring no less than the width of the gap in the fascia.
Press the wood patch into the gap between the fascia boards. Continue adding wood patch until the gap is completely filled. Run the flat edge of a putty knife along the filled gap to create an even surface. Let the wood patch cure for at least six hours.
Gently rub the cured wood patch with fine-grain sandpaper to create a smooth surface along the seams. Wear a face mask or avoid breathing in the dust kicked-up by the sandpaper because it can cause respiratory irritation.
Run a bead of exterior-grade silicone caulk along the filled-in seam to prevent moisture seepage. Flatten the caulk using a putty knife to give the seam a finished appearance. Allow the caulk to set for 24 hours.
Paint the filled-in seam and the surrounding area so it matches the rest of the fascia. Use exterior-grade paint to further protect your home from moisture damage.