Having wound the odd tfmr with HV secs I've found it's essential that you treat the wire with kid gloves so the enamel doesn't get damaged,you need to make sure there are no kinks too. This is easier said than done if your winding a very high voltage low current winding IE thin gauge wire, go up a gauge or two if you have the space to allow for losses due to resistance, it's also easier to wind.
The problem when a novice winds a tfmr is finding out some of the specs of the materials your using & EHT materials are very specialist and usually V expensive. IE you'll find no voltage/insulation breakdown specs for tfmr copper wire, only heat and chemical specs. Try to use class H poly wotnot coated wire. I use kapton tape for insulation,it sticks well and is thin.
I'm about to wind a 1000VA 1500v toroid and have looked into things a bit, but I'll still be winging it to an extent. I'll wind several smaller voltage secondary windings, say about 170v AC making the windings aren't too tight and I'll coat with air dry tfmr vanish as I go, I'll also be testing as I wind. I'll be testing for pri/sec insulation as well as insulation between each wndg and each turn up to 10Kv.
If your going to use an EI core it might be best to use a bigger than needed core and bobbin, calculations no matter how carefully done have a habit of being wrong. Your first tfmr wind probably won't go to plan, you haven't five tfmr winds say, under your belt; any job is part calculation,part experience and gut feeling.
Lastly if your rewinding an old tfmr with a primary already wound you can work out if the core is roughly suitable by saying your efficiancy will be 90%, IE 90% of it's VA rating. Check the wire gauge of the primary to be sure with a micrometer, there are lots of amps, resistance per gauge X on line, use the "enclosed" "chassis" specs, not free air obviously. If you use an un-speced tfmr, IE no VA rating there is a free windows simple tfmr calculation software,see here -
https://320volt.com/en/transformer-c...cin-hesaplama/ that'll work this out for you. A toroidal tfmr is good here, as there is no max space as on a bobbin and you can work out an unknown toroid core by weighing it and doubling it,then looking up commercial tfmrs of the same weight.
Am sure there are a few tings I've missed,oh yes....using an unknown tfmr core of laminations is always a gamble as you have no BH curves, that said you can still wind a decent tfmr for a non critical application and wind custom jobs with the specs you want ,with as good as if not better specs for less money than a commercial jobbie.
Good luck, Andy.
PS, I have a few scope tfmr's if decide not to bother.