Above is part of my weathering kit. (I also have some grey pastel/chaulks and a couple of jars of powders which I used, depending on the job). They are packs B and C of the Tamiya weathering packs - I have looked at A and D but decided there wasn't much call for them. I have also used Humbrol 113 on a dead, dry brush, to put frame dirt on Farish shiny plastic chassis.
This is the subject - my TPM FNA wagon with a sample wagon to copy from which is on page 87 of "British Railway Goods Wagons in Colour 1960-2003 - For the Modeller and Historian" by Robert Hendry. There are other photos on Paul Bartlett's excellent site - see link to the side here and then put FNA into the search box.
The base model has been finished as best as I can in a paint colour close to what it will end up - in my case Humbrol 63 "Sand". There was advice that Desert Sand was a better match but I was unable to obtain it and I made do. Weathering hopefully will get the shade closer to the photo.
Application is very straight forward using the Tamiya kit. I have obtained (via Amity) some extra bits to put the colour on - from the eye wear section of the local chemist. Simply rub the pad into the chosen colour, then apply to the item being weathered. In this case, I used the dirt colour at the right hand end of the B pack. I also used some of the white in the same pack to pick out the corners like in the photo.
Basically my advice is to do it small and slow - build up the colour. The advantage with the Tamiya stuff is it comes off with a baby wipe if you don't like it (wet ones et al - ours is the local supermarket chain brand) without touching the work underneath (which since it has been matt varnished should be the case!). It is important to remember that most wagons spend more time stationary than travelling and as such, the rain runs should reflect this - go from top to bottom, rather than side to side. However, I find when doing planked wagons, it is important to start off side to side as this gets the colour in the grooves and then you can go down (another tip on this is watch out for rain strips on rooves - the weathering is either cleaner, a different shade or darker at the end of the rain strip).
Above all, find a picture of your subject - or something similar if your particular thing wasn't well photographed. Colour is naturally the best but obviously hard to come by for those modelling pre-Grouping or indeed Grouping layouts. No excuses for the Post-Privatisation scene however.
No photos of the finished article I am afraid - didn't get around to it today as I had to change two wheels on the car owing to a pair of flat tyres. No, I only have one spare so there was much to be done in between. Photo tomorrow hopefully.
One I did earlier - I did the 37 and BG last year for Swallow Street using the Tamiya kit and the Humbrol paint on the dry brush