Well, my method is a bit different from most. I'm not too comfortable working with these "new-fangled" acrylic paints, so I use Testor's white (solvent) enamel paint and a bottle of the thinner/brush cleaner, and a very fine paintbrush - sable, preferably.
I first set the wall flat on a level surface. I dip just the very tip of the brush in the unthinned paint, then dip the brush in the thinner, so it's loaded. Then I touch the brush carefully to a mortar line. Capillary action spreads the thinned paint along the mortar lines, without getting it on the brick surfaces. Repeat as necessary, moving around the wall, until the lines are covered evenly, without being really heavily done.
Too much mortar is even more unrealistic than none at all, IMHO - after all, some building owners actually paint their brick walls a brick color to seal the brick and mortar from moisture - so mortar lines should be somewhat understated. On a real brick wall, you generally can hardly see mortar lines from 200-300 feet away...
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Gary M. Collins
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"Common Sense, Ain't!" -- G. M. Collins
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