I have used chemical coolant-system sealers. K-W Permanent Metallic sealer has kept my Olds 455 from peeing it's coolant for about ten years now, maybe more. (Porous weld repair to the cylinder head.)
On the whole, they're not my first choice. A core plug may leak because it wasn't installed properly. You seal the leak, the system pressurizes...and the core plug blows-out. Or it may have a pinhole actually leaking, but is severely corroded on the back side. You seal the leak, the system develops pressure...and the core plug ruptures.
The same sort of thing can happen with other cooling system parts--failing radiators, failing heater cores, failing coolant tubes, failing head or intake-manifold gaskets. You seal the leak, and cause a catastrophic failure later.
Given a choice, I'd prefer to identify the true cause of the leak, and FIX it, rather than dropping sealer in the system. A sealer may be appropriate, but only when you know why the thing is leaking to begin with.
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