Exactly. The formula is always approximate, and use whatever works for you, but I use this:
Darkish: 1 part ink/6 parts water
Lighter: 1 part ink/10 parts water
Just enough to make it a little duller: 1 part ink/15 parts water
Black: 1 part ink
If you want your ink to remain really dark, but flow into every crack, thin with alcohol. This technique does work for washes, but it makes your ink flow away from surfaces and into the cracks. It is best for inking seams, armor joints, cracked surfaces, etc. You can just touch the tip of an ink loaded brush to a joint and the ink will literally flow into the joint. It is not the best for washes or glazes.
For washes and glazes I thin my inks with a mix of 50/50 water and Future floor wax (clear acrylic liquid wax), and add a generous amount of flow enhancer. I use Liquitex Flow Aid. This makes your inks very smooth and flow into all the recesses. You can also use the same thinning formula with paints to make washes. With paints I also add a paint extender. It increases the drying time of acrylics so you can blend easier and the paint has time to flow before drying. Helps to eliminate any water spots. I'm currently using Folk Art Extender. I get both the flow enhancer and extender from hobby stores that carry acrylic paints, Michaels and Hobby Lobby in the US.
I havent been inking long, but I usually take around 2 drops of water with one drop of ink. It works fine for me. But, I only use blue ink(For my GK) and it makes the armor have a nice shiny new look to it.
I add flow enhancer to every mix I use no matter how thick or thin. However, to airbrush acrylics they need to be very thinned. The problem is too much water raises the surface tension and reduces the paint's ability to adhere to the model. For airbrushing I add flow enhancer, but I swtch to thinning with Liquitex Airbrush Medium. You can make the paint as thin as you want. If you don't mind the slightly higher expense of of the airbrush medium over the 50/50 future/water mix, you can use the airbrush medium all the time.
On a side note, Liquitex makes all kinds of mediums for acrylics. You can get gloss, matt, crackle, texture, thickeners, blending mediums and more.
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