Mortar

This is a household latex paint thinned to the thickness of milk with water.   A couple application may be necessary.  Eventually, may be, a light sanding of the surface to restore  brick color. 

This was painted with a Krylon spray paint (?? Forest green??)  before a brick wash was added. 

Each panel should be detailed, laying flat on the work bench, before the building is assembled, IMO.  Mike CT. 

I've done them with the latex paint, it goes very quickly and looks really good.  You can vary the look from old brick to new brick by how fast you wipe off the paint.

I have been doing mortar on brick building models for 22 years and still haven't found a method that consistently gives uniform mortar coverage and predictable results with which I'm satisfied. It always seems to be hit and miss with different results every time. I usually use acrylic paints (off white, concrete) thinned with water but have also used thinned lightweight spackle wiped off immediately. First picture is an Ameritowne plastic kit. Second is laser-cut bricks into fiberboard.

MELGAR

I am a big fan of the joint compound method.  I build up my brick structures before I paint them.  This allows me to blend the corners so there is no tell-tail seems.  The joint compound doesn't require the model to lie flat, so I can knock out all sides at once. 

These shots are scratch built using styrene brick sheets.  Coloring is spray paint and brushed on craft paints.  The craft paint lays down nicely if I use flat spray paint or seal with Dullcoat.  After the cleaned up joint compound has fully dried, a again seal with Dullcoat. 

MELGAR posted:

I have been doing mortar on brick building models for 22 years and still haven't found a method that consistently gives uniform mortar coverage and predictable results with which I'm satisfied. It always seems to be hit and miss with different results every time

You had to go and specify consistently!

I like to use window glaze, colored with paint, and buffed with 0000 steelwool.

I tried some Age-It Easy over the joint compound. The one on the left looks better but both are pretty dark. Guess if one likes the real dirty factory wall look this stuff may work. This was full strength out of the bottle. Perhaps if one tried a dry brush technique?