Looking good so far Henri, all look like they've been worked hard on a real railway
I had a go at weathering an old OO gauge body (it had bravely laid down its chassis for another project) with artist's oil paints. There's no relief detail from rivets or anything on the tank sides so I was wondering how effectively I could actually get grime onto the surface in a convincing way. The major difference found with oils is how long they take to dry. It can be useful in that it gives you a lot of time to rework the surface or remove it all while it's still wet. However it means that you have to be really careful handling it for a day or so after. The extra time to dry and the ease with which it can be removed made me think this could be quite a useful way to start trying to add muck to some locos.
The tools required: 1 old brush for application; 1 nicer soft brush for working the paint; a jar of white spirits and oil paints. What I also used but forgot to include is cotton ear buds.
Firstly the colours are splodged onto the surface. I was expecting the grey to be a lighter shade than it actually was, in reality it wasn't much brighter than the satin black of the tanks. This is using the old brush, as it is not kind to the bristles. I used the brown around the edges, where if it was lighter, it might have highlighted some of the nooks and crannies around the edges of the tanks like trapped dirt. The grey was concentrated on the bulk of the surface to tone down the black.
Using a soft flat brush, a small amount of white spirit (or other thinners) was brushed onto the surface, streaking the oil paints in the down the surface. The same technique was used on the tops of the tanks, except here more pooling occurs. It can be seen on the footplate edge at the bottom of the tank, how this causes the paint to build up.
After this the bulk of the paint is removed using a cotton bud. I would probably use a tissue or a kitchen towel for something G-scale. Again this process starts at the top and works vertically down, right from the top of the tank to the bottom in each pass. Not too much of the colour remains but it leaves some streaking and takes away the flatness and the cleanliness of the surface. Once I had done the whole surface and it had dried over a couple of hours I used a clean dry bud to buff away the paint over the crest and number, like on a few photos I had seen where the crew had wiped the number clean.
Finally after everything had spent a day drying I buffed the lot up with a large soft brush to remove the hard edges on the streaks. Some lighter grey acrylics were dry brushed on to highlight where the tank fillers might spill over the top. I think the whole effort might have been more pronounced if I started with lighter colours, the overall effect is quite subtle and the camera seems to struggle to pick it up. Comparing it with the unaltered side (and where I hadn't done anything to the bolier) I think shows what i was trying to achieve, not a locomotive at the end of it's life but that works daily and isn't cleaned. Just enough to add some texture to the big flat sides and to stop the satin black looking clean, polished and new. Once i'm a little happier with the technique the 2MT in the background will be the first working loco to receive the same treatment.
The untouched side: