Use a powered wire wheel brush, putty knife, or scraper to remove large chunks of adhesive.

Alternatively, spray the entire surface with Xylene or other solvent that dissolves adhesive, let it soak for ten to fifteen minutes and the adhesive will peel off with minimal effort.

Remove any remaining residue with a fine wire wheel brush or coarse (50~100) grit sander. The final surface should feel smooth.

Clean the surface with some isopropyl alcohol.