Remove any paint from the interior of the brick wall by brushing a moderate layer of paint remover over the brick surface. Wait 10 minutes for the remover to penetrate the paint, then scrape the remover and paint away with a putty knife. Wipe paint from any hard to reach crevices in the wall with a piece of cloth.
Scrub the wall clean of any residue with a scrub brush and a pH-neutral cleaner. Use a concrete degreaser on any portion of the wall with an oil stain present. Rinse the cleaner from the wall with water, then pat the brick surface dry.
Search the wall for any visible cracks in the mortar. Fill the cracks with joint repair compound. Use a putty knife to pack the cracks to the surface of the mortar, then smooth the compound out. Allow the compound to dry overnight.
Fill a hand-held pump sprayer with brick and masonry sealer. Pump the handle of the sprayer three or four times to increase the internal air pressure. Point the nozzle of the sprayer at the wall, then cover the wall with a moderate coating of the sealant, stopping just short of saturating the surface with the liquid.
Roll the sealant evenly over the bricks with a low-nap paint roller. Roll the wall twice, once in a back-and-forth motion running the length of the wall, then in an up-and-down motion, covering the wall height. Allow the sealant to dry overnight, then apply a second time, following the same coverage methods.