Haven't varnished a bass yet, but with the other instruments I'm using a ground of some sort, which varies depending on what my zodiac tells me

. But right now it's shellac ground, very thin. next comes a rubbed coat of varnish mixed with pumice. Again, very thin. Those two are my ground coat, and thin enough to almost look like it's polished, with no coating on it. After that comes either glazes, or colored varnish, or a combo of the two. I try to keep the varnish/color coats to two coats, max. then sometimes a coat of clear, or nothing, or some times a french polish as a clear coat. My system varies a lot, it's flexible because sometimes I antique, some times I don't.
My color glazes are oil colors, I dont use browns, but make browns by mixing transparent red, yellows, and blues. The glazing technique that works for me right now is to mix my glazing oils with some of the varnish, to make a paste.I'm finding that the closer the color is to the wood, the more intense it can be made, as opposed to trying to get a deeper looking color with, say ,the fouth coat of varnish. I haven't had much luck going that way.
I like wood texture so I'm always trying to keep things a thin as possible.A lot of my varnish coats are rubed on and patted down, as opposed to brushing. And my varnishes come from either violinvarnish.com or magister varnish, out of Holland. I'm not brave enough to cook my own, and I'm not convinced I could make any thing better than what's out there.
But they are crazy expensive, so that's why I'm curious about the rock hard varnish as an alternative for basses.