Thanks all.
Even if I stripped it all and took it to a sandblaster or got a mobile crew around, they aren't going to get at this rust.
As you see in the pics above, which are a before and after pic of a bit of the worst area, the chassis is made up of 2 lengths of C section steel, approximately 1/4" thick that fit together and are riveted through the vertical face.
The rust flakes that have formed have spread the lips of the C section and they remain spread once cleaned out. I used 2 foxing wedges, (blue thing in the crack in the second pic), to spread the lips then hammer on the external horizontal face which causes the layers of flakes to spring loose. I flick them out then use compressed air to remove any remaining rust dust and chunks. The steel is really tough and it takes a lot of work with the 4lb hammer to drive the wedges in and they often pop back out when I am hammering on the chassis. Even where I can get the wedge all the way in, it only opens the gap about 10mm. When I drive a wedge in, it only opens a section about 40mm long before needing a second wedge. I would need over a dozen wedges just for the bottom of the chassis rail and even then, it would only about half the length needed. With such a narrow gap, I doubt sandblasting would do much.
At work, I share a building with a group who do materials protection management. They monitor contractors doing cleaning and recoating of things like bridges, tanks and the pipes and tunnels for the hydro stations. They also test and recommend suitable protective coatings for industrial situations. I explained what I had been doing and how it didn't work. I mentioned that I was considering cleaning it all again, as best I could with the hammer and scraper, then using Rust Converter. They said it was a total waste of time. Their recommendation was to clean it as I suggested, flush it with thinners or acetone to remove any dust and moisture, and once it has dried out, mix up some 2-part Zinc-Epoxy primer and work my way along, spreading a section and forcing the mix in and squishing it around with my flat scraper so it covers both faces. I have to run the scraper along, like spreading icing with a spatula, which will remove excess primer, so I can later clamp the lips back together and tack weld them. The primer has a long working time, compared to car bog or fibreglass resin, so I don't have to worry about it going off too fast. Their other suggestion was to pump a good thick bead of sealant into the gap before clamping and welding, to keep moisture out, but that is just as likely to trap moisture in, as I can't seal the entire top, bottom and end seams of the chassis without totally dismantling the truck. (Although if I had a big, weather-tight shed, I might consider it).
On another note, I pulled a couple of the bolts that hold the bogie axle assembly to the chassis and found that some of the original bolts had been replaced, as there is a mix of metric, imperial, coarse and fine threads. The original spec bolts were Grade 5, 5/8" UNF bolts, 16 at 1-3/4" and 12 at 2".
A couple of the ones through the side of the chassis rail showed signs of wear, as if the axle assembly had been moving and the bolt rubbed where it passed through the double chassis rail. I called into our local industrial bolt pirates to pick up replacements today, by when I got home I found the 5 in the top of the box were perfect, with a section of plain shank before the thread, and the other 25 under the packet of nylocs were all thread, like the originals in the truck. Then I noticed that the 25 all thread ones were also UNC, not UNF. Too close to closing time. From now on, no matter how busy they are, I'm going to tip the damn things out on the bench and check them all.
I also found out my turps gun is properly called a degreasing or cleaning gun.
I'll pick one up during the week.
If I get anything done, I'll update the thread.
Greg.