Is it possible to improve the airtightness of a ductwork after completion?
To improve the air tightness of existing air ducts, a method of sealing the ducts by applying a sealant using a spray atomizer was developed in the USA in the early 2000s and is now (since 2015) being used in Europe. This method  can reduce leakage in an installed air duct system by 66-86% and can therefore improve the air tightness after installation. This can have other positive side effects, such as the impact on energy costs, indoor air quality & comfort or hygienic requirements.
 MEZ-AEROSEAL. https://www.mez-technik.de/en/mez-aeroseal.html