Soldered track has its advantages and disadvantages. The latter include the lack of chair detail and the need to gap the copper surface of the sleepers to isolate the rails from each other. Despite these negatives, I prefer soldered track for its durability, relative ease of fabrication and overall look that, with a lot of help from Martin Wynne's Templot design software, is a great improvement on ready made OO gauge track from Peco, etc.

The lack of chair detail I can put up with. The gaps have to be filled. The last few days have seen some experimentation on what material to use to fill those gaps. My first plan was to use Isopon filler, bought from Halfords. This is the classic "bondo" that I used when I was restoring classic and vinatge cars back in the 1970s. I don't remember having a separate hardener with the old stuff, though and getting the right proportion of hardener proved to be difficult with the new improved product. So I tried something else.

This was Milliput filler. Again, the product promised much but in my puttying hands did not deliver quite the effect I was looking for and some of the fillings pulled out with minimal sanding.

Back to the tool box, then. I found an unopened tube of the French equivalent of Polyfilla. It's called Semin Rebouchage et Lissage and is specifically for interior and exterior use, including humid areas. It is much finer grained than even fine-grained Polyfilla and comes out of the tube like thick cream.

And it works! Good adhesion, easy sanding and a very smooth finish.

Edit to correct spelling of Milliput!