Am aged mechanic I knew who maintained large, stationary, emergency generators always topped up his radiators with boiled water.
Boiled water is 'flat' - all the natural air has been boiled off - and avoids 'burping'.
Never, ever, try to open any radiator that is too hot to hold with a hand. Water, under pressure, holds air. When you remove the filler cap, the absorbed air will come out of the water and try to escape - squirting wet air and water over your face.
Draping a rag over a cap when opening will catch all the emitted water.
