A friend put in stop leak and now the heater core is blocked. Unfortunately
it is much harder to replace than radiator or thermostat (which seem to be
okay anyway). He put some detergent in the coolant and reversed the hoses,
which achieved nothing. Look through other forums, I see various ideas:
1) put vinegar or tartaric acid into radiator, warm up engine then rev it to
4000 RPM. Then quickly flush system with water and refill with correct coolant.
2) disconnect heater hoses, and pump gasoline into it. Then flush heater with
water.

I gather that trying to blow it out with compressed air is bad, as this may
dry out the stop leak, making it worse.

Has anybody succeeded with clearing a clogged heater?