Washes are a quick way to add a lot of depth variation to the finish of your model. It is usually one of the first stages for weathering and can turn something that looks like a painted piece of plastic into something more true to life. Washes will bring out all of the small details in a model by flowing into those areas and providing a contrast with base paint layer.
I'm not writing this post to describe the best wash or the best technique for washes - they all have their merits and uses and it's up to you to determine what the best tool for the job is. This post is simply my experience using various washes and is in no way comprehensive.
India Ink Wash
This is a relatively new one to me. India ink is ink for pens. It is very strong and stains every surface it touches, so be careful when mixing it. The mixing ratio depends on how dark you want the wash to be. I start at a 10:1 mix. There are a few different things you can mix it with, but it seems that airbrush cleaner works best. I've used rubbing alcohol and water too, but these tend to pool up on the surface instead of brushing on evenly. Airbrush cleaner breaks the surface tension and prevents this from happening. I'd imagine a drop of dish detergent would work to but I haven't tried.
I use this wash to make a nice clean surface look like its been used. It is great for floors and walls. It looks very dark when you first apply it but it will lighten as it dries. It flows into crevices and panel lines and will highlight small details. This type of wash works magic on chrome pieces too, giving it a tarnished look. I have found that this wash is better for larger scale models - 1/72 and larger. I wouldn't recommend using this wash over an entire model, but in targeted areas.
Be careful when applying this straight onto the paint, especially if you are using an acrylic paint, you don't want to strip off the paint. Obviously this will depend on what you used to thin the ink with. My first go with alcohol ended in disaster.
This was is perfect for instant weathering. It comes in several different colors and is easy to use. Its perfect for adding dirt and grime to your model in one pass. It is a water based wash with fine clay (I think) particles, so you are basically putting dirt on your model. And it looks great. Apply it with a brush, let it dry, then rub it off. You can rub it off with some elbow grease and a dry cloth for heavy weathering or you can use a damp cloth and a light touch for less intense weathering. Because it is water based cleaning up your mistakes is a breeze.
There are a few ways to apply this. I recommend clear coating first, especially if you are using acrylic paint. If you use a flat clear coat it will grab the particles in the wash and keep them, making the surface look very worn and dirty. If you use a gloss coat the wash can be easily wiped off only leaving it in the panel lines and recesses. I recommend experimenting with these washes, you can get a lot of great weathering effects with this product.
I love using oil paints to weather models. Their best feature is both a plus and a minus, and that is that they take a long time to dry. I bought a pack of two dozen colors off Amazon. I also have some Mig Abteilung 502 Oils and they are fantastic. They make colors targeted at modelers. My favorite, Starship filth, is a great dark grey/brown color perfect for general I want this to look old weathering.
Oil paint's can be applied directly to acrylic paints or over a clear coat, again depending on the final look you want. The glossier the finish the more easily the wash can be tubed off.
I use oil washes in two ways, and I hesitate calling one method a wash. The first method is to heavily thing the oil paint and apply it like you would any other wash. The second method is to directly apply the oil paint directly to the model then rubbing it off with a cloth. I use this when building unpainted Gundam models, it ads a layer of grime that makes straight plastic no longer look like plastic. You can thin anywhere between the two as well. I use a lightly thinned engine oil color on older engines. When the paint dries it looks like old oil caked on engines or in an engine compartment.
Oil paints are extremely versatile when weathering a model. This post covers washes so I won't get into it here, but oil paints can be used all over models to add tiny weathering details.
I have never used a premade wash but I wanted to include them on this list. The only reason that I haven't used them is that I never felt the need to buy any. From what I've seen they are all pretty good. I might pick up a few in the future, but for now I find it easier to mix my own. Mig makes a line of enamel washed that look good, and they come in a lot of colors.
I have used the Tamiya Panel Line Accent as a wash. It is a very thin enamel product that comes in four colors. It is easily cleaned up with mineral spirits. I love these panel line accents and use them in every build. The black color was the primary wash on the recent KFIR C7 build.
For a good example of almost all of these washes use check out the Revell K100 gallery. I used the Flory Model wash on the cab, oil washes almost everywhere, especially on the chassis and engine, and the India ink wash on parts of the cab and the chrome pieces.