Fluoride varnish or remineralizing pastes can be used after whitening to reinforce enamel

https://sergiojedw518.almoheet-travel.com/the-fact-about-charcoal-tooth-paste-does-it-actually-whiten

Fluoride varnish or remineralizing pastes can be applied after whitening to enhance enamel and decrease level of sensitivity while maintaining your brighter smile.