Leister welding tip

I use metal pipe cleaners and a wire brush. If you do it while the tip is hot, all the crud comes out easier.

 

I don't know what's on the tip or if the weld is vinyl, but Acetone will dissolve vinyl.

 

I don't think it's strong enough or affects vinyl, but stronger acid then attacks the metal. I cleaned cement off a magnesium float with some strong acid but the metal pitted, like craters, sure did clean the cement residue, turned into trash.

 

Wire brush on a drill or die grinder. Sounds like you need more power not chemistry.

 

If you get the big stuff out when it's hot you can use a dremel tool with a brush like the ones on the bottom right in the picture. They will fit through the tip.