These products work on the chemical reaction they have upon exposure to air.

No air, no sealing.

And they can't seal large areas because they don't have the ability to fill in an area, only to harden up when they make contact with the atmosphere.

If your radiator is full of gunk in the bottom then this will just be more gunk. But if you get enough gunk down there the water cannot circulate. This is a big problem on radiators that have the tank on both ends and not top and bottom. With a 'both ends' radiator it takes only a little gunk to starve off your bottom hose.

I have know people who used this in radiators with both metal and plastic tanks and it has worked like it should. One guy who didn't want to replace a $300 radiator just kept a few bottles in his car and would toss in one every time the plastic tank would start cracking.

I finally pulled one for him at a wrecking yard and installed it.

He still carries bottles of Bars Leak with him. Just in case.