For any persistent / intractable scald, especially in the current wet/muddy conditions, we use Stockholm tar. Clean the foot thoroughly, apply the blue spray and let it dry, then apply the tar and let it dry. It seems to keep the foot clean and dry long enough for healing to occur.
Otherwise, giving the sheep some hard core to walk over / loaf on, and (unless it's going to get washed away within minutes), spread lime on well-used tracks, in gateways, around water troughs, etc.
And longer term... Start to keep track of which sheep and which ewe's lambs rarely hobble, which seem to shake it off within a day or two without treatment, which are needing treatment again and again. Keep ewe lambs from the former, stop breeding from the latter. It makes an enormous difference over time.
Rotational grazing helps too, with feet as well as worms. We graze cattle, then sheep then ponies, then rest to regrow to a length which suits cattle, repeat. If that's not an option for you, try to have the ewes lamb onto cleanish ground and move onto clean ground once all lambed. (The ground being cleaned by haymaking the previous year if you don't have any other grazing species.) And think about giving pasture a few months empty after a group of sheep with foot issues have been using it.