Hi Ross
Thanks for your comments. Oil washes are really easy and a lot of fun. First of all, it's essential that the model has a good coat of clear acrylic (either matt or gloss, doesn't matter). This is because the sludge oil wash is mineral turps based. You do not want to ruin your paint job. I use artist oils - pick them up from any art shop. The main oils I use are Raw Umber (dark brown), Panes Grey, white and sometimes a yellow. To show fade, dust, etc, I apply little dots of the oil paint all over the upper surface. Next you blend in these dots removing most of the paint using cotton buds that are slightly damp with turps. You just move the cotton buds over the model in the desired direction. Look at how dust and grim sits on your car and how rain makes it streak down the side and blotch the paint work on the horizontal surfaces. These "filtering" techniques help break up what can be bland surfaces.
Next comes the sludge wash. I mix raw umber and pains grey with turps until I get the desired degree of "thinness". When it is nice and thin, it's great for panel lines. When it's denser, it's great for sludge washing wheel, gun and engine bays. Once you apply the sludge wash, let it dry a little. Do not worry if you've slopped too much about as your slightly damp cotton bud with wipe away however much you like. Once you have removed as much of the oil wash as you like, simply seal this in by giving your model an overall spray of acrylic clear (matt, semi-gloss, whatever).
To me, once the camo paint goes down, this is just the first step or layer if you like. The model itself is like a painting and the overall finish built up in layers.
Here's a great tutorial I found showing oil wash techniques.
https://www.youtube.com/watch?v=LfsFGZbiDLMHere's my oiliest model - A-1J. This aeroplane was without a doubt the oiliest aeroplane ever and nicknamed the "Self-lubrating aeroplane" as it could spit out something like 6-7 gallons of oil per hour! So, I went to town on it - in reference to a photo of the actual aeroplane.
Give it a go on a older model - it's great fun and makes your model come alive! The limit is just how worn and dirty you want to make your model look. My "hero" and greatest inspiration in this regard is a UK modeller called Jamie Haggo (his day job being a pilot in the RAF). His models are hyper-real I think. Here's one of his examples.