There are various meanings behind sealing wood. Some of it depends upon which stage you’re in with your sign while other definitions have to do with the true meaning of the word.
When working with fine woods and cabinetry, you seal wood so it doesn’t drink up so much paint when spraying. Spraying is the best way to give nice smooth finishes to any surface. This sealant closes up all of the porous fibers in the wood, thus allowing nice finishes on furniture and other fine wooden products. The sealant you put on a deck or siding is just a waterproofing stain of some sort. It’s only good for a year or two and then has to be re-applied. The sealing I believe you’re talking about is what most people in this industry refer to as clear coating. It’s usually a poly urethane paint and is applied to bare woods to prolong the life of a sign if bare wood is being exposed. However, you’re still going to have to come back and give it a facelift every few years. No paint or clear coat will protect for long periods of time… especially when you consider what part of the world or country you’re located. All of these paints and clears today breathe at a different rate than your wood and peeling is bound to happen eventually. So, follow up your sign project with a ‘Maintenance Program’ every two years. It’s kind of like an insurance policy for their sign and you get paid to keep it in top notch condition.