The problem is more that the sand trapped in the core mostly stays in the core. You’ll stir it up with a flush and may get some of it out but a lot of it will settle back into the core.
Good luck!
THE Craziest thing I EVER witnessed was when I last worked at a Chevy dealer in the 90's. A woman bought a newly designed Chevrolet Lumina Sedan. She was on her 5th heater core within the first 4000 miles of owning it.
She, rightfully so, was very disappointed in the car, and was on the verge of using the "Lemon Law" for relief.
GM had her bring the car to our dealer and sent a team from "STG" (Service Technology Group). These were mobile engineers who were THE best of the best of the best. (Consummate nerds)!
A team of 4 of them flew here to Houston, and converged on the car akin to a person being prodded by scientific brainiacs!
I watched them from across the aisle and curiously came over and asked questions. They were able to determine that electrolosis was the culprit, not by the coolant, but by the air going across the heater core being ionized by the blower motor! The company (vendor) was Siemens. The fix that day was to go to the home improvement store, buy a small amount of metal window screen material, cut it to the face profile of the new heater core, attach a wire to it ang ground the other end of the wire, and put the car back on the road again, and write a report and get it to Siemens' engineering department!