These are products that are applied as the first layer on the hull surface before proceeding with painting or antifouling application.
In particular, the primary function of the primer is adhesion and surface preparation. The undercoat, on the other hand, serves to hide imperfections, defects, and irregularities of the hull surface, creating a smooth and uniform surface for the final painting.

Essentially, these products allow for better adhesion of subsequent products, can protect against corrosion and osmosis, and allow for surface isolation, for example, in case of using antifouling that is not compatible with the previous one or of unknown nature.