I avoid all types of primer, because they're just too heavy. I get by with some extra clear coats with zinc stearate mixed in for filling the wood grain. It's more work, but the result is a lighter finish
Primer is not exactly required, but highly recommended, for epoxy finishes over dope subtrate. Otherwise the adhesion is less-than-ideal. In any case, the primer is epoxy clear mixed with - wait for it - zinc stearate - and a different catalyst. And I think the same is true of Brodak dope primer.
The difference is that the epoxy primer (just like the colors) can have a vastly higher ratio of pigment to carrier without adhesion issues than can dope. That's why Windy's method of concentrating the pigment (letting it settle out, then skimming of some of the clear) works or why adding pigment works, but is moderately risky with dope. You can either get adhesion issues or the underlying colors will bleed through. That's also why epoxy covers so much better. The volume of pigment per unit volume of liquid is much, much higher. I would guess that a quart of KlassKote or K&B primer contains about 5x as much filler as a quart of dope. Same with the colors. Pick up a can of Klass-kote White or primer and you would swear it was poured full of lead instead of paint. And it has the consistency of very heavy cream, honey, bordering on pudding.
The key is that you just don't need to put very much on. I did a lot of pretty careful tests on it, and I think to get a similar level of opacity, you need something like 2/3 the mass dope. And that's roughly what you get on completed airplanes - maybe 8-10 oz. of dope and 6-7 ounces of epoxy/car paint to get the same quality of finish, assuming nothing goes wrong.
All the big weight issues I have seen with it have been the result of treating like dope and applying FAR too much. I knew one notable whose name will be omitted (unless he wants to own up to it) that ended up with something like 12 ounces - of CLEAR alone!
Car base coat (which I haven't used, but I know most of the pioneers) seems to be much more like epoxy than dope in the covering power VS mass.
Brett