Engraving enhancer used on this side of the pond from the year dot by gunmakers and the like, not expensive easy to put on and just as easy to remove waterproof as well. Take a visit to your local art suppliers and purchase a bottle of black "India Ink", it is a tried and tested formula Shellac and lamp black easy to apply using a cloth or cotton wool to run into the engraving, then let it dry (usually about two minutes) then go over again with a cloth slightly moistened with Alcohol to remove from the high spots. Apply and remove until you have the effect you are looking for, also it can be touched up any time with no problem.