I seem to remember reading that during WWII there was an Air Ministry specification on oatmeal for aircraft engine cooling systems. (I think it was supposed to cure ordinary leaks rather than bullet holes!) Given that English mustard powder is largely very finely ground flour it looks like it wasn’t just someone flying a kite! (Sorry!)
Erik’s remark about some preparations clogging small waterways could have some foundation. In order to block a hole you have first to span it and then the “plug” has to harden, presumably then the particles in these preparations could span an already restricted waterway. What is unlikely to happen is that the stoppage will harden, as most preparations seem to require access to heat and air for this to occur. If this is the case as long as the radiator is not allowed to dry out and/or get hot it should be possible to flush it out.
If it is suspected that harder deposits such as lime-scale restrict the internal passages then it might be worth trying a domestic kettle cleaner to dissolve out contaminants before trying a sealer. If kettle cleaner fails then there are more aggressive cleaners used on boiler systems, domestic and industrial.
Looking at the Scott cooling system, unpressurised, no fan, no forced circulation, the size of the radiator and the engine’s output, (as much heat as horsepower remember), its ability to reject the waste heat must be pretty marginal even with everything perfect. If the radiator is so bad/old/abused that you are terrified of finishing up with sieve or a pile of crumbs after a good cleanout then maybe it is time to start saving for a new one!
The trouble with radiators is that they are largely made of brass.
The trouble with brass is that it is an easy material to work, it’s cheap, has an attractive colour, takes a nice shine and doesn’t go “rusty” in the way iron & steel does. On the down side a major component of brass is Zinc and zinc can and does dissolve out. On an old radiator I’ll lay odds this is the major source of troubles, the dissolved zinc leaves microscopic cavities and cracks at the grain boundaries. The material is now no longer a cohesive structure and becomes brittle; the damaged area is also a seed point, aided by a bit of vibration, for larger fatigue cracks. Just to make matters worse these microscopic cracks make it almost impossible to get the brass chemically clean so that when you do try to solder up a leak the blankety blank solder won’t stick.
All of which leads me to wonder if German Silver or Monel metal might not be a better bet.
Or even air cooling…
Take no notice, my other bike’s a Comet.
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