Disclaimer: this is from my experience painting for a couple years. Aside from some videos and feedback on my work, I would consider myself self taught. I HIGHLY encourage you to take what works for you from each persons advice, and make your own process that works for you.
I have only only used Army Painter (the dropper bottles, not the cans) and GW brand washes, and agree that they are not created equal. Army Painter is my preferred choice, because I can thin those with water, without needing to purchase extra supplies. I bought GW expecting the same, and I was very much mistaken.
To me, washes can be used for many many different things.
All washes are going to settle in the recesses to make detail pop, they are also going to effect the color of the paint. They will take something that looks like a kids toy, and turn it into decent tabletop quality almost on their own.
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Washing is is almost always my last painting step before varnish. I don’t know that I have ever highlighted afterwards, because I feel I don’t have the time or the skill. I also feel I can get similar effects with a drybrush before washing.
Lots of people will wash wash the entire piece in the same wash and be done. Usually using the dip method that can be found online. This is great if you need to get it done quickly.
Even at this stage, it’s important to understand how the wash you use will effect the color of the paint. And plan your paint colors accordingly. It gets more important once you get into using multiple different washes on a piece. Colored washes, especially, will augment that same tone in a paint. (Check out the troll below for an example of adding a green wash to a paint with slight green tint)
Even when using multiple washes, I tend to go back and run a light wash over the whole piece, to help blend everything together.
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The body of the troll above was all painted the same color. I used a green wash over everything but the stomach. I then used a thin layer of a light brown to blend it together.
Also,don’t be afraid to experiment with putting colored washes on paints that don’t match. My favorite zombie skin tone has been putting a green wash over brown paint for a rotted look.
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And sometimes you get happy accidents. Like this flesh wash over Army Painter’s toxic boils.
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(Original color on Flayed Man. Bolton is after wash)
So I again encourage everyone to experiment and take the bits and pieces that work for them. In the end the only thing that matters is “are you happy with your results?”