Yes, for cable I'd recommend borax from the start. It's a complex dirty weld with stuff being burnt off and stuff being squeezed out- you're fortunate to get clean welds all the way through anyway.
However, it DOES help to soak your cable section in kerosene for a while, perhaps over night, before heating fluxing and welding. I think the kero acts as a solvent to cut some of the grease in the cable, and does not itself form a barrier to welding or even to further fluxing with borax. I think that the grease to be found in cable often acts as a hydrocarbon flux itself, but I can't see it hurting to thin that out a bit and wash the cable of any grit etc by the use of kero.
I've not tried this myself, although I'd like to- the use of agricultural lime as a flux. Heard that one a few times over the years...
And Stacy missed one, dry welding (no flux) without first welding up the seams. This has become popular in the last few years, although it's been done by a few such as Bob Kramer for some time. This consists of making a billet in the standard fashion, tacked together at the ends and likely a bead down the sides, with a handle welded on... the forge pre-heated and dialed to a reducing atmosphere (a few inches of orange dragon breath) and the billet put right in with no kero, flux, whatever. You need a soak time of 5 min or so after achieving welding heat throughout, and then it's right into the press and crush it all shut first heat. After this I'll often brush the sides and add a bit of borax before back in for the second heat, to reduce the amount of little black lines to be ground out from the sides from oxy hitting the seams on the way to the press. It's not a big deal, I just like to waste as little as possible.
This can be done with the mill scale even left on the pieces first, or so many reports would have it, if the initial soak time is increased to 10 minutes or more in a reducing atmosphere. I've not tried this, as I really don't care for the possibility of flaws in a billet. It's not that hard to grind mill scale off when building a billet.
I absolutely recommend this "bareback welding" as J.D. Smith calls it, as it yields the most clean perfect steel inside a billet. You'll consistently get blades that are indistinguishable from monosteel before etch.
