A very acceptable way to start that process would be to apply a coat of Roberts Brick and Mortar Formula. This will fill in the mortar lines and you can keep as much of it on or just rub off to a level you like. Its a white chalky liquid material you paint on with a brush and you can rub off the excess. You can leave it like that or seal it and use other weathering. I got a jar of it at my hobby shop but I bet you can find it on line as well.