David wrote:
Is there something in between (perhaps 60W or more) which will give enough
local heat or am I scuppered because the ring part conducts the heat away
too quickly?

Notwithstanding the issue about melting or damaging the inserts, I think
you'd probably OK to solder the rings with a 45W iron. Maybe a little on
the low side, but should be OK.

What's you're missing is flux. The surface is probably dirty, maybe
painted. You need to get back to clean metal so that solder flows on the
surface. You don't melt solder on the iron, you heat the metal and feed
metal onto the surface. The only purpose of solder on the bit of the iron
is to ensure good heat conductivity from the bit to the workpiece.

Likewise with a gas torch, you don't wave the solder into the flame, you
heat up the metal and pool molten solder on the surface. However, I think a
gas torch is way overpowered here and difficult to control.

Flux is the key to getting solder to stick to metal, particularly oxidised
metal. You can use plumber's flux, but liquid flux is less viscous and
easier to apply thinly:
https://www.ebay.co.uk/itm/Electrolu...e/130962877320

The symptom of not-enough-heat is when solder on the bit solidifies when you
touch the workpiece, and stays solidified no matter how long you hold it, or
if you can melt solder on the workpiece close to the bit but not further
away. If you can melt solder on the bit but not on the workpiece, you have
a heat conductivity problem. If it doesn't stick, you have a flux problem.
(or have non-solderable metals)

Theo