Hi Trevor,
There is a whole set of those pens and as Graeme says the are like the Rotring and other makes. They have a wire down the centre so there is very little space - only enough for ink.
The list of them is at the following link - where they are the top item.
http://www.proopsbrothers.com/acatalog/Online_Catalogue_Lubrication_32.html < Link To http://www.proopsbrothers...ue_Lubrication_32.html
Graeme's link is to a proper lining pen which has a larger reservoir; another maker is 'Bob Moore' which looks to be the same - that uses neat Humbrol enamel.
I could not get the link from Graeme to work - if you have the same trouble goto
http://www.peterspoerermodelengineers.com/
then to Easy Liner.
These are modified from a very old design that preceded the tubular pens.
Here is some more advice re that pen - mainly for 00 gauge but there is a larger nib.
http://www.rmweb.co.uk/community/index.php?/topic/7441-bob-moores-lining-pen/ < Link To http://www.rmweb.co.uk/co...bob-moores-lining-pen/
http://www.rmweb.co.uk/community/index.php/topic/1149-bow-pens/ < Link To http://www.rmweb.co.uk/co...p/topic/1149-bow-pens/
The usual way is by the old fashioned bowpen - which is now very hard to find one of the best makes is a 'Kern' which is swiss.
There is an article on painting & lining from the Manchester model railway society. that may help you here is a link -
http://www.mmrs.org.uk/technical/painting.htm
That article gives advice on the (Humbrol) paint and its viscosity.
Hope the above helps
Lining is a 'black' art I cheat and use transfers!
All that is for 'lining out' - for what you want to do there is now a paint from Model Masters, called # brick filler' which means the gaps between the bricks!
http://www.modelmates.co.uk/en-UK/Opaque_Brick_Filler/Opaque_Joint_Filler.html < Link To http://www.modelmates.co....aque_Joint_Filler.html
Again for the smaller scales; so a mix of acrylic paints to the consistency of milk (as you want some density) should work - let it dry as it will dry lighter than the darker colour when wet. If not dense enough add more apint (and some windscreen washer liquid to kill surface tension, and try again, wipe it off the bricks surface when its half dry, using a cloth and gently.
A bow pen could possibly lay it down the grooves but the vertical joins will take an age and send your eyes dizzy!
If you are quick acrylic paint can scrubbed off plastic quite well. When you have your mortay in place varnish it to keep it there! the you can paint the bricks - in many colours.
Finally I have heard of baking soda being used: perhaps for our large scale mixed/fixed with varnish (sprayed?) For interest re this and brickwork in general have a look at
http://trainweb.org/crocon/pcoquet/scratchbuild_article.html < Link To http://trainweb.org/croco...atchbuild_article.html
again its the smaller scales, (S scale this time) and the photos on the left are clickable to larger versions.n
Have fun!
Yours peter