my pot of gunge was built up over time. I started when I was running shop, and was painting stuff with all sorts of colour. I had 3 pots, one started as clean water, and when that got too dirty , it became pot number 2, and pot 2 was pored into pot 3. pot 3 gradually got more gungy, and I used it for wet weathering. Messy, but gravity and water takes rust effect to where it actually would be. If colour was not quite right i just dabbed a spot of different coloured acryllic or emulsion paint and then either more very dilute gunge of plain water. Gravity did the rest. Messy .
I now remember I did not tend to leave brushes in water, as have found that eventually destroys them. I found that dipping paint brush gummed up with dry pain, in bottle of liquid plastic glue(the plastruct one was always to hand), actually cleaned brushes, but probably also shortened life of brush. Glue might not have been as good, but I had enough spare. One reason I only buy cheap paint brushes.
For me there is no such thing as real prototype colour. It is what looks right then it is right, and even brick red or slate grey are different on different buildings. I sometimes wonder if it is sales people pushing products just to get sales, but then some people seem to prefer to use what they are told to use. Having said that, if i as only doing a few one jobs then a set of weathering paints might be useful.
I find it even easier with my 3D printed models as any water based works. For proper plastic, I only need to prime plastic with a preferably flat grey acryllic paint, then use matchpots of emulsion paint bought from my local Wilco store.
For examples of how I apply weathering then pictures of my buildings are best as I do more with buildings and they photograph better. One on my building is to the side of messages here. I am not saying my method is better, but I think people should try out different methods to see what suits them, and you can not lose my trying something for free.
Narrow gauge Francophile interested in 1m, 60cm,50cm , 40cm and smaller gauges . Build in scales from 1/6th to 1/24th. Also 1/32nd and 1/35th using 16.5mm track to represent 50cm and 60cm gauges. http://www.rue-d-etropal.com