Consider this...
You have a small crack in your plastic radiator tank. You decide to apply JB Weld, or an epoxy, to the crack (on the outside of the plastic tank). When the engine and cooling system goes from cold to normal operating temperature the system pressurizes. Our vehicles use a 16lb radiator cap so the hot coolant pressure pushes outward on a crack that has been patched from the outside. I'd say the odds of this type of repair working are slim and if your driving and the crack widens and lets coolant leak out without you realizing it the systems pressure will suddenly drastically increase as the the coolant temp begins to boil, not to mention the inconvenience of you will now likely will be left stranded somewhere.
I might be making some assumptions, since the average person would already assume it needs replaced.
Those assumptions could be something like that there is a need to continue using the vehicle, or money is tight, or COVID has caused a supply problem for parts.
This within the context that the vehicle has already gone a month without an overheating situation presenting itself, so the regular driving routine is not overheating it, and the awareness of the problem, suggests to me that someone who is aware, and has a couple gallons of water onboard, could go a fairly long time with a radiator top crack.
If it overheats, yes you pull over. It is not going to explode the engine right away to have the coolant boil, rather if one is aware this coolant issues exists, they can be watching the dash temperature gauge. They would not be stranded to the extent of leaving the vehicle there, merely needing to let the engine cool down a little and if low on coolant, add some. Just sayin', if it came to it, that beats walking to work till you have the chance to fix it right.
I don't think the crack widening makes that much difference. You could do this with the radiator cap off. It'd just spew coolant all over the engine bay if you kept driving once it started boiling. Maybe I'm thinking of a different sensor system but can't we even hook an OBDII dongle up and have realtime coolant temperature info to see if it's ramping up fast?
It's not ideal, but I don't think it is the problem you're making it out to be, if you monitor it. I've rode in several vehicles that were worse off than that back in the day. It might be a Mad Max scenario but you do what you gotta do...
As far as applying epoxy on the outside of the crack, I suppose I didn't elaborate enough. The idea would be to take a plastic squeegee (credit card, etc) on a bead of epoxy and push it down into the crack, scuff a significantly larger area than just a few millimeters on each side of the crack, and if you happened to have a piece of fiberglass mat or cloth, inlay that into the epoxy too, but, I was only talking about a short term repair till a new radiator can be installed.
If it breaks on the first time out for more than 10 minutes, it was little ventured, nothing lost if you just pull over if it overheats. I guess I just don't see being stranded on the side of public roads for 20 minutes to cool down, as the hardship it used to be, now that everyone has a cellphone.
