Personally I'm not a fan of the various additives meant to plug leaks and cracks, out of the suspicion that if they plug a leak, they can also plug up a needed coolant passageway as well, especially if the cooing system is not flushed and replaced on a regular basis. Plus I'm not sure of the chemistry of those things. BMW is rather fastidious about what things shouldn't be in the cooling system, and I imagine that applies to other manufacturers as well, the intent being to avoid untoward chemical reactions with the materials in modern engines and cooling systems that could lead to corrosion, gumming up the system, formation of sediments, etc., etc., etc.
My advice is to replace the radiator (and this would be a good time to replace some hoses as well) with a new one (obviously), but I don't think it needs to be OEM, that's unnecessarily expensive. I don't have any specific brands to recommend as replacements, but I imagine that a decent service shop would know the good and better priced ones, and perhaps some other members might chime in. Sure this would cost a few hundred for the job, but in return you'd be getting a peace of mind that "pour and pray" couldn't give you.