The above picture shows two side pieces of a Walthers H.O. roundhouse kit. The upper piece is shown as it comes in the box with brickwork stamped into its face. The rather bright colour was toned down to a more realistic brick tone by an aerosol spray coat of red oxide primer. Some like to fill in the mortar lines with brushed on diluted white acrylic paint which is then quickly wiped off before it can dry on the bricks themselves. The picture below shows another method using drywall goop to make the mortar lines, a wee bit messy but easier than paint to remove with water if you don't like the end result.
After smearing on the goop and working it into all brick areas, you can start wiping the excess off just as you do with paint but timing isn't critical. If you let it dry, you can still wipe the excess off with a damp rag with good results. An X-acto knife with a No.11 or chisel blade will quickly remove unwanted goop from creases or edges.
Lastly buff up the surface with a clean rag to ensure that all brick surfaces look fairly even in colour although all real-time brick surfaces have colour variations in them.
