[AAWSCC]

I had a tin of magnetic paint which I used on the Pioneer test track I built last year. The paint 'went off' good enough and we painted over with household water based emulsion without any problems. I would say you should leave it to dry for 24 - 48 hours before painting over and I agree with the others that sealing it now will just seal in the uncured paint.
The really annoying bit is that the magnetic paint doesn't actually give any effect. Fine if you are making a kitchen notice board with the mags directly on to the board, but no good for slot cars with a gap. We even tried adding iron fillings from a local steel works to 'beef' up the paint.

I would imagine that you have a bumpy surface to tape to as well, unless you masked off where the copper tape will go. I always tape before painting. That way you get the copper and the track surface at the same level.

 

[AAWSCC]

Don't take this the wrong way MR.F, but I find it hard to see that it's the paint making the difference in your video. Where the car has better traction is where the rails are close together so you have the extra thickness of the tape and the lack of MDF/paint because the car is over the slots. I would put that down to one car having better tyres than the other.
Plus testing with a battery directly onto the copper is not the most consistant way to find out.
We put 6 coats with extra iron fillings in the paint and still not much magnetic effect.

 

[AAWSCC]

Hope I didn't offend MR F. That was not the intention.
I was just trying to give my opinion.

As for the suggestion of using steel braid, yes I agree that would give you a lot better magnet effect, but I think, like me, it's a cost thing. Copper tape is so much cheaper.