My first question to the manufacturer would be, why do they not want the inside sealed. Possibly they have weep holes which need access to the inside.
GEII is a great easy to use sealant. Many people think you need gobs to do a good job, not so, a minimal amount works great and looks great. Apply it so you have a minimal amount to wipe off as you smooth it. Denatured alcohol makes a good pre-cleaner, after you have any scum removed.