Is it worthwhile spraying any liquid inside the sills to slow down rust that can be seen on the heater channels and if so, what do I use? Cavity wax? Deoderised fishoil? Lanolin? Anything else? Or am I wasting my time.
Cheers

its worth doing
I would wait for a hot day then drill a hole in the back of the sill so you can flush out all the dirt n dust from the inside with water
allow to dry then spray in Cavity Wax
you have to remove all the loose stuff first other wise its just going to continue to rust

Also remove the running boards and back seat interior panels using a hacksaw black and water again flush out the rear quarter panel using the blade up the drain holes to make sure there unblocked

The front quarter panels also hold dirt and rust make the small hole bigger in the bottom of panel n flush out with water

also using rust converter is a good idea as no paint in any of the areas

I think Wurth has good product it will smell for while but use the heaters for few days

early bugs were dipped in a gal type primer ? meaning inside of channels etc and not sure if aussie ones were from ( ca 1960!) So yes, drilling holes also the size of a good rubber drain plug will be good. then fit it -
the ones where water gets out but not easily in..(funnel shape...)

We sell a lot of techtyl to car resto customers. Some mix it with fish oil too.

Thanks folks. Once the weather warms up (10 degrees in Melbourne atm) I will get some wax and give it a go.
Cheers