I once had a recipe for a do-it-yourself job.

It was used on a hunters barn .22 and his foul weather shotgun, so you may prefer a coating more professional....

But, it was basically flat black auto-primer that was thinned with mineral spirits and then applied in as many as 6-7 coats, allowing full drying in between coats. Then, finally, several coats of "Brite" floor polish were applied followed by smearing on paste wax and leaving the wax coating unpolished.

I myself never used this process but kept the article figuring I'd have a cabin or truck beater .22 or shotgun someday.

If I can dig out the article I'll scan it and post it here.