Here's another useful tutorial from Martin Kováč, enjoy!
Areas of collected moisture, often referred to as Wet Effects, along with leaking or spilled grease and fuel, are one of the finishing touches that are very enjoyable to create.
Murky Water is a glossy enamel product that has a slight brown tint. It’s recommended to thin it down a little bit with Enamel Thinner before adding it to your model.
You can apply it using a fine brush creating controlled amounts of water streaks on vertical surfaces. This effect enhances the appearance of previously applied dust tones and creates exciting contrast with their matt finish. Try to keep the amount of the product loaded onto your brush minimal if you want to create very fine streaks. You can apply the same mixture in a cloudy pattern over horizontal surfaces and enhance the effect with speckling.
Grease and Old Grease are some of the more recent releases from Wilder. One of them has more of a dark brown color, while the other is more black. They can be used in layers, starting with the lighter Grease effect. It’s also preferable to thin them down a bit if you want to achieve less opaque results.
Application of these effects is the same as in the previous example, but they produce a different, much darker and more apparent finish. Apply them more sparingly as they can become a bit overwhelming if applied harshly. Start with the Grease effect and then fill the inside of larger patches with Old Grease. This way you can achieve deeper effects which are more interesting to look at.