I should think it would be fine kept in an airtight container - given a good shake & mix before next use !
4 largish spoonfuls of cornflour to a pint of water - cooked, has been a nice consistency for textured paste - adding poster paint once the paste has cooled. Good for holding bits of collaging materials in place as well.
Is this about the same as cornflour paint?
Arrowroot has the same thickening qualities as in some jellies / Fruit flan gels - haven't looked for it in a long time though !
Let us know how the practice runs go