Priming a wood surface is almost always a good idea before applying an opaque finish, such as opaque lacquers or paint. If you don’t prime the wood, the first layer or two of end will end up serving as your primer anyhow, and utilizing primer in the first place is more effective. Shellac primer works well enough under most situations, but also has the virtue of being the major primer to be used over previously stained wood. With no shellac, you will need to remove that stain until you refinish the wood.
Fill any cracks or deep gouges in the wood surface with wood putty, using the putty knife.
Sand the surface of the wood to prepare it for priming, using an orbital sander to the big distances and an oscillating tool for corners, borders and inaccessible spots. Use rough, 60-grit sandpaper to eliminate rough spots as well as out dents. Switch to 120-grit sandpaper for sanding the entire surface, and smooth things out with 240-grit sandpaper.
Apply pigmented shellac to the wood surface with a paintbrush, using brief and even strokes.