In the mid seventies I painted my newly built harpsichord in three colors with alkyd paint. Six coats with sanding between. It took about a month. After the final coats, it took several grades of wet sanding followed by rubbing compound.

Then there's composite gliders. Gelcoat is sprayed into molds before layers of fiberglass, Kevlar, or carbon fiber alternated with adhesive is laid into the mold.

Gelcoat durability is problematic. Eventually you have to sand WITHOUT damaging base layers before repainting.

Composite adhesives can be toxic. I know a canoe builder and a glider repair engineer who died prematurely – and two harpsichord builders.