To treat breastplate rubs you can use the Green Ointment https://www.cheval-energy.com/en/complement-alimentaire-chevaux-derme-epiderme/225-green-ointment-greenpex.html .
You can apply the ointment in prevention before working, or you can use it to treat friction, after working.
It is THE miracle ointment to treat the small wounds of the everyday life.
If the friction is related to a hunting collar, for example, do not hesitate to add protections in sheepskin on your harness, such as protections for halter https://www.cheval-energy.com/en/equipement-cheval/ 459-protection-halter-sheep-genuine-performance.html? Search_query = sheep & results = 20 , that will fit perfectly and reduce friction.