Using an underpainting and painting a gradient on top of another, with gouache

Question

Premise:

When we paint with gouache, previously laid layers of paint will get re-wet, and dissolve again, mixing into and strongly impacting (usually, for the worse) whatever we are trying to create on top.

I know one advice is applying layers in a specific order: starting with very diluted, and using less and less diluted subsequent layers: starting with "watery" and progressing towards "buttery" consistency.

Yet I'm afraid even this layering discipline won't help sufficiently when I'm:

• trying to use an underpainting
• trying to create a gradient (I mean "blending") (which, with gouache, is challenging already in itself) on top of a previously laid layer of a gradient.

Casein? Very close, but...

I know James Gurney likes to use casein, which will not dissolve once it dried. He uses it as underpainting under gouache pieces, and for entire works too.

But: casein does not seem to be widely available in my region (and I'm not big into online ordering paints), and anyways I'm also discouraged by the challenges that its organic nature imposes on handling it (like its quite short shelf-life). So I would rather look elsewhere.

What other approach can one take?

What other thing could I do, what additional material(s) should I bring in to achieve my above introduced goals?

I am interested in solutions that:

• do not compromise the archivalness of the finished works: I wish uncompromised lightfastness (as good as the original paints'), and reliable, long-term adhesion to the carrier material (paper, gesso'd panel, etc.)
• (preferably) leave the painting's surface similar in character to the original gouache's: I might still would like to apply some color pencil strokes on top, as part of the final steps

Answer 1

• Use acrylic paint as an underpainting. It can be safely used in combination with gouache. To retain as much as possible of the character of a gouache painting (the matte look and texture), I suggest diluting it nevertheless, which, in addition, will help keep an eventual underdrawing visible and improves adhesion (based on the discussion here, it otherwise seems best for adhesion to use a matte medium over the acrylics before continuing with the gouache, or to add gum arabic to your gouache). Please do some tests with this in advance, though: too much dilution will compromise the binding properties of the acrylic.

  Shelf-life is not optimal, but better than casein and tempera, and you can cover your palette/dish/container with cellophane to effectively slow down the drying process whenever you take a break.
  Acrylic is flexible even after drying, and can be safely used on paper, canvas, and panel.

• Use tempera to create your underpainting. After drying, it will be insoluble. It is known for its durability. During the renaissance, even after losing its popularity, this was still regularly used as underpainting for oil paintings because of this characteristic.
  On a layer of tempera it's easier to create gradients using gouache.
  Egg tempera is also fairly easy to make, and it won't interfere with the appearance of the gouache that's applied on top of it.

• Another way to keep your underdrawing intact when working on panel, and without having to resort to (organic) mediums, is by engraving or incising your linework. You can also indent lines on non-hardened gesso. You will be able to see the depressed lines even after several layers of thin paint, but of course they might remain
  Of course, this technique is dependent on your artistic style.

Answer 2

There is also acrylic based gouache available, which will not lift after it has dried. You can use the acrylic gouache as an underpainting that way without it lifting.