high voltage coil protection with epoxy resin
Not enough info.

What "high voltage?" 500V? 200KV?  Are your windings on a plastic spool?  Are you embedding the core as well (and why iron, rather than ferrite for high-freq?)

Many-kilovolts HV protection is usually via silicone rubber casting with hardner (2-part silicone liquid,) not epoxy.  Can't remove epoxy to inspect a failed coil, at least not easily.

Look up the "dielectric" rating for your epoxy, volts/mil breakdown, etc.   Buy "high-dielectric" epoxy.   Or get castable liquid silicone rubber for HV encapsulation.