Johnson's tinning compound..........
This is my first post on this particular gun website, so be gentle.
There is a liquid compound that is sold as "Johnson's Tinning Compound". Been around forever and used mostly in auto body work. It's chief claim to fame is that it is absolutely guaranteed to get solder to adhere to steel. I've used it to secure Mauser rear sight bases and it works just fine.
I might be off on that name as it's been a few years, but any decent auto supply store should be able to locate it. I'd suggest trying it on a few scrap pieces of steel first to get the "hang" of it.