I have rust treated my 90 series a few times now since I bought it 7/8 years ago, got the sills welded / patched when first bought it, then I have been cleaning and painting it every 2nd/3rd yr since. Had to get them same patches re-welded (done by myself and a mate this time) around yr and half ago along with some new patches. All in the sills except for 1 or 2 patches towards the very rear of the chasis frame. I done a good big job on it that time, I cleaned it all down to clean metal, removed fuel tank, etc,,, grinded / welded / painted with Owatrol rust preventitive liquid, and then painted 2 coats of Owatrol black paint. Was a good weeks work during lockdown.
I was looking under it last weekend and it seems to have held up well, a lot of the paint is gone off the sills but there are no new rust holes, just surface rust I think, I'm going to clean up the just the sills again now and re-paint them as they seem to be the main weak point on mine, my chasis is very good, but to be honest it's becoming a pain in the balls having to keep on top of it every 2 yrs or so.
Is there really a "job" you can do on it that'll last 10 yrs. I'm not convinced, the majority of folk saying they got such treatment to last 10 yrs had it applied to a much newer vehicle where it wouldn't have rusted anyway. Mine is 24 yrs old and I think at this age your looking at re-treating it every 2/3 yrs max.
I have avoided wax-oyl and Schultz such products except for on wheel arches as I was told if the finish cracks and water gets behind it it will trap it and rot it so I have always went for the more laboursome painting method thinking it was a better job but I'm not sure now. I might try the Schultz on the sills this time round. That epoxy paint mentioned above sounds interesting if it will last longer, not sure whats involved in applying it though.