Layers isolation in a Toroidal Core
It needs to be silicon steel - this increases the electrical resistance to about 4 times higher than normal iron. This and the fact that laminations are insulated from each other to minimize eddy currents means you have an uphill struggle.

Regards varnishing, this is usually necessary because the laminates need to be insulated from each other to prevent significantly worsening eddy current losses - without varnish (or treatment to produce oxides) the core will act as a small resistance single turn shorted secondary.

Useful Link here

And another here