rain scald is found in a copper deficient host.
Easiest way to clear it up is a wash in a 30% copper sulphate solution, and put 1tsp of copper sulphate in their feeds daily for 3-4 days (dissolve the bluestone in water obviously)
Once you get it cleared up, then yes, feeding something like seaweed meal or Himalayan Crystal Salt is generally enough to keep their copper levels up (free feeding it obviously)
I used to wash the bullmastiff in the copper sulphate solution too at first sign of his rain scald type eczema (except his would progress to huge open weeping sores within 12 hours of him getting wet, so a little worse than the horses rain scald they generally get)
Unfortunately we don't usually have 2 troughs available to horses but with dogs it's easy to have 2 lots of water out for them and put a copper pipe in one lot. They'll drink it when they need it

(haven't done this for mine they get the salt LOL)