I recently got some zinc plant markers with metal spikes and zinc plates, and you write on it with a carbon pencil. To my displeasure, the day after I put them in the ground and watered the garden, some of the labels got dirt and/or dust splashed on them, from the rocks and gravel in the garden. And it made the zinc plate look dirty and the writing got hard to read. Unfortunately when I tried to wipe them off or clean them I discovered they were permanently marred and also the attempts to clean them only smeared the writing.

So I did an experiment. Some of the zinc labels were luckily not ruined because they were shielded by other plants so they didn't get dirt on them- so I sprayed those zinc labels with a silicone waterproofing called Camp Dry (used for tents and boots, etc). I gave it two light coats of spray, a few hours apart. When dry, I put them in the ground and watered the garden. Magically the water and splashed dust/dirt rolled right off the label, and it stayed clean and beautiful. Also the silicone spray added contrast to the writing so it was darker and easier to read.

After continuing the experiment I have found that the writing and clean look of the labels that were sprayed with silicone waterproofing have remained beautiful after several waterings.

Hope this tip helps some other gardeners!