It would help

Youtube is your friend ...

I'm not sure what product you have there, but it sounds rather thick. That may be just the ticket, actually, if this YouTube video is any indication ... you don't want super thin and runny with this method:

Remember, YouTube is your friend when you need to find some how-to-do-it demonstrations like this.

I'm sure some may jump on here and start lecturing us on how "you can't see mortar lines on real brick structures when viewed from a distance" ... but reality is it varies. There is no one-size-fits-all with modeling brick -- real world structures vary as to how thick the mortar lines are and how visible they are from a distance.

Here's a few examples ... sometimes the mortar lines show up, other times they don't.

Spackling Compound

I've used a spackling compound made for patching nail holes in your walls before painting. It is a damp but almost crumbly consistency out of the container. I have rubbed it into the bricks with my fingers and then used a damp rag to clean the excess off of the brick surface leaving a residue in the cracks to represent the mortar. Works very well. I have even tinted a portion of the product with craft paint to get a darker mortar and applied the same way.

Sounds like what he has. I did not watch the video before posting this.

Espee over Donner

I just use plain old Plaster

I just use plain old Plaster powder which most every one has. Just rub it on the brush off with a soft brush then I set it with cheap hair spray. here is a N scale sample

rich 

Mortar lines on HO

Mortar lines on HO  structures are TINY...I have used a watered down craft acrylic white, light grey, or tan ish...watery thin!! 2 to 3 goats let air dry drag a damp brush or paper towel across the face of the bricks avoiding the mortar lines to remove the grout like haze of left behind paint...Goggle Jason Jensen trains on You Tube, he has used watered down joint compound on wood and hydrocal structures...Honestly anything thinned enough to set or dry in the mortar lines look great... good luck.. 

My go to for mortar is joint

My go to for mortar is joint compound.

Joint compound

I have found that joint compound is my preferred method as well.

It is thicker than paint and seems to grip the mortar joints better.

In Australia, the brand I use is CSR Gyprock Joint Compound.

Apply all over the brick wall with a small artist's spatula working it into the mortar joints and scrap off the majority of excess compound.

Let the joint compound go off for around 10 mins before wiping off the brickwork gently a few times with a damp cloth, waiting to let each wipe dry before wiping again.

Don't try to get it all off in one go.

Gary

GREAT!

Unfortunately, I don't remember the name of the product and the tub (tiny size - 1/2 oz.) has no label, so I was shooting in the dark. Your suggestions are good ones for me to try. Many of the "kits" come out in bright red bricks that stand out like sore thumbs which is why l posted this.

The Greater Nickel Plate will have its first shakedown session on May 2nd to find and work out bugs as scenery continues. The big problem will be any track realignment I haven't spotted yet and making sure all the rolling stock and engines meet coupler heights and weights. There are 28 locomotives and more than 250 freight and passenger cars to check. Given everything was in a U-rent storage locker for 6 months till last December, I want to be sure any gremlins didn't creep in.

Thanks for the suggestions. I check here every week to catch up on ideas for improvements to my railroad.

Dave C.

Chief Cook and Bottle Washer

Join coumpound

I achieved this in N scale with join coumpound

The base is a Plastruct brick wall, painted with Golden acrylics paint

The stone part is a Kibri plastic stone wall painted individualy with Golden paints

The wall when dry is dry brushed with a mixture of india ink and isoprpyl acohol

paint red plastic brick red

First painting red plastic brick with a rattle can of red paint is usually worth doing.  The red paint gives a slightly rougher surface which contributes to realism.  As "mortar" is wiped off, some will adhere to the brick surface giving it a bit more texture.  Weathering chalks are an additional option for heightened accents.

I use joint compound applied with a damp sponge and then wipe a clean damp sponge across the surface to reveal the bricks/stone.  While I work in 1:48 scale the joint compound has no grit and has the consistency of sour cream.  It should work well for any scale.  One advantage of joint compound is that you can go back after it has dried and rework it with a damp sponge or paint brush to to finesse areas to your satisfaction.  I use it straight out of the tub with no thinning.
- Tom