I think it may even be just the flux, certainly look slike it as most pastes containing solder are grey. PDF on Silverline site looks like it is zinc chloride and no mention of any metal content.

You could silver solder it though it is not easy to CI as you need to drive the carbon off the surface first. I'd whip out the JB Weld and stick a bit of over thickness steel to the casting and then machine it.

Edited By JasonB on 21/04/2019 17:45:05

Rik

I had a similar problem a few months ago with a cyl. casting for a ST #1. The side of the valve face opposite the exhaust port had a large chunk knocked out of it well into the valve face. I machined it back by about 3/32" and using JB Weld fastened on a piece of 1/8" steel held on by 2 countersunk screws with the heads well below the surface, and filled with JB Weld. Then machined the surface back to the correct dimension. Once painted the fix will be invisible.

John