If you live where it EVER freezes, the water that is still trapped in the brick will freeze, which will systematically destroy the bricks. Before you do ANY SANDBLASTING, consult a brickmason in your area and get some first hand professional advice. You could be creating a HUGE problem for yourself. Brick, especially older brick like yours, is baked in an oven. The outside is harder than the inside, so sandblasting could be very risky.