The maestro uses a Sharpie® brand metallic paint marker to sign — its vibrant, oil-based ink adheres particularly well to the polyurethane varnish used on our instruments, and is reliably durable, water and fade-resistant.
Other than ensuring that it is not exposed to direct sunlight for prolonged periods, there should be no need to protect the signature if the guitar has been purchased for display purposes.
However, being signed on the body's upper bout, the signature will almost certainly wear off during playing.
We therefore suggest that you arrange for an acrylic encapsulating lacquer coat to be to be applied to the top of the guitar as soon as possible after delivery. We strongly recommend that this work is carried out by a reputable and experienced professional custom guitar or paint specialist.
Alternatively, for a cheap and cheerful solution, you can apply some durable clear self adhesive film (or iPad/tablet screen protector) over the signed area, with all corners rounded so there are no sharp edges where the film can catch and peel off with playing.