Nothing comes back out
A pot of wash is a one-way street. You can add paint, medium or thinner to it; you cannot remove any of them. That makes correcting a mix a different problem from making one, and a much less forgiving one, because the target ratio has to be reached by raising every component up to its share rather than by adjusting the one that is wrong.
The defaults show it. Six millilitres mixed at 1 to 1 to 2 holds 1.5 ml of paint, 1.5 of medium and 3 of thinner. To land at 1 to 2 to 7, the paint has to end up at a tenth of the mix — and there is already 1.5 ml of it. A tenth of the final batch has to be 1.5 ml, so the final batch is 15 ml. You add nothing to the paint, 1.5 ml of medium and 7.5 ml of thinner, and you finish with two and a half times the wash you were trying to rescue.
The binding component is not always the obvious one
It is whichever component sits furthest ahead of its target share, measured as the volume you have divided by the fraction the target wants. In the default case that is the paint, at 1.5 divided by 0.1, giving 15. Medium gives 7.5 and thinner gives 4.29. The largest of the three wins, because a batch smaller than that would need paint taken out.
This is why a mix that is slightly too thin is cheap to fix and a mix that is too strong is expensive. Too thin means the thinner is binding, and thinner is the cheapest thing in the pot. Too strong means the paint is binding, and every millilitre of excess paint drags seven of thinner and two of medium along behind it.
Dilution steps get smaller
A 1 to 2 to 7 wash is 10 percent paint. Cut it one to one and it is 5 percent, cut it again and it is 2.5. In absolute terms the first halving removed five points of pigment and the second removed two and a half. The amount of pigment removed by each successive halving shrinks the same way, so the pots at the far end of a dilution series differ from each other by less and less. Whether that difference is visible on a model is a separate question the arithmetic has no view on.
Parts are only as good as what measures them
Every ratio here is by volume. A part measured as a drop is a part measured with an instrument that changes with the bottle, the tip, the temperature and how thick the contents are. A drop of thinner from a pipette and a drop of paint from a dropper bottle can differ by half again, which turns a 1 to 9 wash into something between 1 to 7 and 1 to 12 without anybody doing anything wrong.
The drop counts on this page are there because drops are how people actually work, not because they are accurate. If a mix has to come out the same next month, measure it with a syringe in millilitres and write the millilitres down.
What decides whether a wash works
Not the ratio. Surface tension, the varnish underneath, how matt it is, whether there is flow aid in the mix, how fast the carrier evaporates and whether the pigment stays suspended for long enough to reach the recess. A calculator can hand you 10 percent paint by volume; whether 10 percent looks right over your particular basecoat is settled on a test model.
Questions people ask
How do I fix a wash I mixed too strong?
By adding, since nothing can be taken out, and the paint already in the pot then sets the size of the finished batch. Six millilitres at 1 to 1 to 2 contains 1.5 ml of paint; if the target wants paint at a tenth of the mix, the batch has to finish at 15 ml. You add 1.5 ml of medium and 7.5 ml of thinner and end up with two and a half times what you started with.
What does a 1 to 9 wash mean?
One part paint to nine parts of everything else by volume, which is 10 percent paint. The page splits the nine between medium and thinner because most wash recipes use both, and reports the paint fraction so you can compare recipes that quote the ratio differently. All of it is by volume and none of it says anything about pigment load, which varies between paints.
How many drops are in a millilitre?
Whatever your bottle gives, which is why the field asks. Count how many drops it takes to fill a 1 ml syringe and divide. Tips differ between manufacturers, thick paint drops larger than thin thinner, and the same bottle changes as it empties. A ratio quoted in drops is only repeatable when every component comes from a comparable dropper.
Why does diluting further stop making a difference?
Because each halving removes half of what is left, not a fixed amount. Ten percent paint goes to five, then two and a half. The first step removes five points of pigment, the second two and a half, the third just over one. Past two or three halvings the pots stop looking different on the model, which is the practical limit of the range.
Is it worth correcting a mix or starting again?
The page gives you the number rather than the opinion. Correcting the default mix means finishing with 15 ml when you wanted less, and the added material is mostly thinner, which is cheap. Where it stops being worth it is when the correction produces far more wash than you will use before it separates or dries out — in which case keeping the strong mix for another job and mixing fresh is the cheaper move.