How can I stop paint lifting on radiator?

There isn't really anything I personally know of specifically made to seal lifted paint on radiator. I would advise that you sand down the affected area and it's surrounding and repaint that area feathering in to the existing paint for a smooth finish. The sanding will allow the paint to have something to adhere to. A quick little trick that I have used personally, which works wonders in sealing and protecting the paint on the radiator, is to use a High Temp Clear Lacquer once the paint has fully dried and settled. They usually come in spray form.

peel as much off as possible until it stops peeling sand down the edges, then apply 70% paint 30% water mixed, allow to dry then apply your top coat

You need special radiator paint heat resistant for rads

Likewise, I would advise that you not only sand back the area but the whole flute of the radiator where it is affected because it will be almost impossible to blend one layer of new paint into an already existing dry layer. You will always see a raised and lower area. Sand it back and repaint at least two layers allowing 16 hours of drying time between if you are using solvent-type paint. Four hours if you are using water-based paint.