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

https://devinjdan182.cavandoragh.org/the-fact-about-charcoal-toothpaste-does-it-actually-whiten

Fluoride varnish or remineralizing pastes can be used after whitening to strengthen enamel and lessen level of sensitivity while maintaining your brighter smile.