So I thought I'd report back on progress....
I tried a lot of the tips/suggestions above, but in general any kind of chemical (stripper, oxalic etc) gave very inconsistent results. The beams had various kinds of accumulated finishes (bits of paint, various stains/varnishes in various colours), some appear to be a different species fresh from the builder's yard complete with random nails causing black marks and some sort of thick green paint that would be more at home on a shed than indoors.
Doing test sections also resulted in an 'overtreated' section that looked worse as it was hard to even up with the adjacent bits when I returned to finish the area.
Of couse, once you start and have made things look worse, you must continue digging.
The process I ended up going through more or less:
- To clean the beams with some water/soda crystals and scrape off any large oddities.
- Once that was done, I basically taped off each room, and hit them with my orbital sander (80, 120, grit). This was pretty laborious waving the sander overhead for hours, and with my old sander generated a lot of dust and vibration. I upgraded to a festool one, and the dust extraction was so much improved that the dust levels were easily dealt with by simply vaccuming the room afterwards. Getting into the edges that meet the ceiling generated the most dust as the small delta sander had much worse extraction with the triangular pads.
- Some parts of the beams had 'character' wear (cracks, deep gouges etc) that would have meant sanding them down so much it would have looked bad, so I just cleaned those up with scrapers/strippers etc and left them in place.
- Finally once they looked 'OK' I went over them with the oxalic acid leaving it 20-30 minutes or so and then washing off with soda crystals in water to deactivate. This cleaned up some of the stains and evened out the various shades somewhat...
So - a painful job, but in the end the sanding was not as dusty as I feared. The main issue was that the vibrations (that were much less into my arms with the festool) were transmitting loudly into next doors house generating lots of complaints, so I ended up scheduling the adjoining rooms were the beams must be resting on a party wall of sorts for days when they were out at work!
Anyway... a few before and afters. Not perfect, but certainly better in our opinion and has lightened up the rooms.
Thanks all for the advice!