Plastic Radiator: Repair or Replace?

1 Generally speaking: Is it standard practice to replace a plastic
tanked radiator or do radiator shops repair these? Say for new cores
etc... I'm kinda thinking in terms of old copper or brass radiators and
don't know what the standard practices are for modern radiators w/
plastic end tanks.

2 Specifically if there is a tiny hairline crack in a plastic tank on
top of the rad that produces a drop of water every 5 or 10 minutes -
can that be 'sealed' say with a special plastic welding process or
perhaps scuffed and use JBWeld or ...?

I'd buy an all brass one. I've never seen anything work on old dirty
plastic. Sealer from inside might / might not / might clog up ???

To expand the crack to clean it will only aggravate problem,
if you must try a fix, Weller makes a " Plastics tip " for the 8200
model gun, a repairman's favorite. You could try to melt in some milk
bottle plastic, A La Ski bottom repair candle.