You could read up on Subaru Coolant conditioner aka Holts Radweld; I have not used the product, but stumbled upon it when the Subaru dealer indicated to my boss that "nothing could be done" about the leaky HG in his car. I mentioned if nothing could be done, why was subaru selling a product that appeared to be something?
His wife ended up with a Mini w/o DI.
edit: this type of solution is offered in lieu of suggesting a new radiator, which is the more prudent fix; Nothing worse than a leaky radiator, and the aforementioned fix is like scotch taping ends of a rope together.