if that rudder were a surfboard, i'd say that was a fabric inlay. the fabric is from the fabric store and is applied and trimmed prior to the glass job.
i don't know how log lasting fabric is if you don't burry it with glass. just a clear coat isn't enough on a surfboard, even if you use rice paper instead of fabric.
these days you can print your design on special paper like rice paper with an inkjet printer and fiberglass that in. the ink can fade tho.
for an existing rudder, sand off about as much material as you expect to add in the new process....good luck with that. your rudder is going to come out heavier and thicker if you add layers. plus the new layers could pop off if the rudder flexes much.