Jim - I think the problem is that he needs to weld the ends in while the tube is out of the vehicle so he can get to all sides to weld. Then he'll weld the tube into the vehicle. So he can't put the oil in prior to welding in the tube, and can't roll it round afterwards.
That's why I think he needs to seal it and put some preservative inside.
I think Jim's thought was to leave big enough holes at both ends (like in one lower corner of each end) so I could put a garden hose against one hole and flush everything out the other end. That might work, but I think I'd rather have it pretty well sealed with some preservative in it.
And although I could get at the ends enough to weld them on after the tube is on the vehicle, that definitely wouldn't be the easy way to do it. And I don't see much benefit to welding them on later, so I'll probably do them off the vehicle.
But I definitely don't want to put the oil in before welding the tubes to the vehicle. Pouring a bunch of heat into tubes filled with oil doesn't sound like a great idea. And the tubes will be welded to the body tub. There's no decent way around that.
That Par-Al-Ketone and also the beeswax sound like stuff that stuff that more needs to be applied. What about just pouring a pint or so of bar oil in each tube? It won't get the upper surfaces very well, but water won't ever pool on that either. Although I suppose enough water in the tube and the oil would float away from the steel so it could rust under the oil but in contact with the water.
Still, I think that will work well enough. So I guess that's how I'm leaning right now.
I was talking to a friend today who is an aircraft mechanic also and he said they have to check a tube every five years and apply either par al ketone or linseed oil.
He said last time they looked it was coated and some of it was still wet after five years. And its not sealed.