Agree w above. Pics would help accurate suggestions. Topically, if the horse has thrush, treating that will help, and if he lives in a wet environ - feet soft because of that, soaking in super strong saline can help a little, though keeping in a dry environ at least most of the time is the better answer there.
No, there's nothing you should paint on to harden them IMO. I personally don't like the idea of painting soles either, but frogs are more permeable & have lots of sensitive areas, proprioceptors, etc, which makes them more easily & seriously damaged with hard chemicals.
Protecting & supporting hooves when necessar, with boots & pads is also important in developing strong hooves, because the more comfortable use, the better they get.
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