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I have no access to. commercial kiln wash so I made my own with alumina hydrate and kaolin. when I mixed it and applied it dries quite quickly and doesn't spread easily. it is very dusty.

is there something I'm. doing wrong

Probably doing nothing wrong. If you add more clay the kiln wash will stick to molds better but it will also stick to glass better.
Kiln wash once dried will be dusty if not much clay and won't stick to the glass so much.
You have to figure out what you want and need and mix accordingly.

There are also different grades of the alumina.
Just remember your lungs are not designed to be filled with fine dust so take safety precautions.

As for not spreading easily, add more water and use multiple multiple coats to build up the layering you need.
Hake brush applied to shelves hot enough that the solution immediately sizzles dry. Repeat.