I am mildly surprised that no one has suggested that your Luger may be a Russian capture/VoPo Luger that was exported to (or more likely through) GB and received the British proofs there. A Search here will reveal instances of a 'tar-like' coating applied to these Lugers, and various methods of removal.
I would go with the crushed walnut shell blasting, or crushed corncobs, as a non-destructive method of removing the coating.
An alternative would be to entrust the Luger refinishing to an established restorer. Gerald Tomek on this forum is highly recommended by many members here for this refinishing.
