Yep, not nice.. but its all down to that part being replaced a few years abo..
That's true Si, wherever bodywork, welding, replacement panels have been done, you can as good as guarantee it's half-life.
Whilst i think Thomas' right about flushing box section out after wading, I think for most its far too late for that.
Once its like his red one, all you are going to do is re-slurry it all and re wet it, taking weeks to dry again, and never wash it all out.
Once really dry, well into summer, I would rod it to loosen it up and knock and vacuum out. I'm pretty sure I've managed this well enough on one of mine and then initially sprayed in there.
Use
thick fencing wire, round the ends off so it doesn't scratch too much, or loop it over. A few different lengths, and a rubber hose (heater hose is about right) on the vacuum wedge tool seems to be the business.
But if there are those large rust flakes in there, like you can see in Thom's photos, then they'll not be coming out and be blocking the vac all the time - then cut the floor open if you're a welder like Thomas.
I'd judge Thomas' to be
extra bad, and as he says, waded, mudplugged a helluva lot. The problem with mine was that it had been once or twice early in life but never cleaned, so went in different areas.
We should really build a rust library - I've got a few hundred photos by now
Si's B pillars, interesting. The door pillar access holes down to that dodgy to re-weld plate inside the jacking point -
always rod around and vacuum that out too, and where sills meet front and rear jacking points, always seem to go there too.
If there's dust/muck/silt anywhere - there'll be DAMP hanging around in winter too...
Behind panels like wheel arches, if you ever see dry or flaky surface rust, jump for joy, 'cos that's
telling you where the problem will occurr - quick wire brush or rotary twist knot, another one of those thowaway brushes (10 for poond, in't £1 shop) and a splosh of Rust Encapsulator at £25/litre! Clean and fill the rear of seams with it too... but must be dry and grease free.