Anything is better than nothing, so if you feel happiest with paint, then use that - but don't forget that you'll still need to connect it to ground somehow in each cavity. The thicker the paint, the more conductive it is, so I'd use more than one coat. I haven't found any scientific testing that says copper (or aluminium) is better than conductive paint, plus the paint is easier to apply. StewMac recommend using conductive paint for cavities and copper tape or foil for the undersides of pickguards where it's easier to get a flat finish.

I'm really not sure how well paint or tape works anyway when the top of the cavity is open and there's no metal cover on the pickup. the shielding will absorb some RF signals but I feel a lot will still get through. The cavities aren't really deep enough or the pickups sitting inside them enough for them to act like true Faraday cages. So here they may attenuate rather than completely block RF signals (though any attenuation is beneficial).

Shielding is more effective in the control cavity where a conductive box can be created, and the smaller wiring holes in it meaning that shielding should be good down to the mid-microwave wavelengths (though some nearby mobile phone signals may still be picked up as they work at these short wavelengths).

So I'd stick with the paint as you've got it. You don't need very low resistance for the shielding to work properly. And just remember that most commercially made guitars don't have any shielding at all!