Unresolved solder won't stick to heating element

Hi guys,

Some time ago I noticed that my printing bed won't head up anymore. While looking at the underside of the bed I noticed that one of the wires was loose. I am trying to repair this by soldering some new wires in place. I did not have any thicker cables so I just used 3 thinner ones to form 1 bigger cable, so far so good. The problem now is that my soldering tin just won't stick to the metal of the heating element (as can be seen in the photo).



I think that the factory solves this by using this red epoxy or some sort of other material. But I don't have that laying around and was wondering if there is another way of fixing this?

With kind regards,

Xander Boomars