What type of dental treatment protects the margin from decay between tooth roots and crowns? Will it last after dental cleanings are done?

Fluoride. for good protection nothing beats proper oral hygiene via good brushing and flossing. If you are still concerned or at a higher risk then had a good fluoride rinse to your daily schedule (ACT fluoride rinse)
Resin Infiltration. is an innovative approach primarily to arrest the progression of non-cavitated dentin with open margins of the ill-fitting crown.
Crown Margins. Good day Harbinder; Marginal adaptation of crowns is fundamental to the durability of the restoration. Precious metals like 18 carat gold lasted a lifetime. The margins of the newest ceramics are not as precise. Scan. Electron Microscopacy would confirm. However, meticulous dental technique, exquisite home care and regular cleanings are critical factors inmaintaining decay free crown margins.