This map represents the typical scholar posters: with its wooden slats to roll and hang, lined on the back. It was very common to varnish them with shellac to waterproof and protect them from abrasion. This one was made of two pieces of printed paper, sticked together along the central horizontal stripe. It is from 1936, spanish civil war was barely breaking.
Removing the old varnish has allowed to repare other minor damages: tears, gaps and wrinkles. But most important is that the new varnish is not oxidizing nor yellowing. As it is very flexible it will not crack in the future.