If you apply the spray correctly, then you’re going to struggle removing the mark. The goal behind these types of products is the permanence aspect of it. That can present a problem if the mark doesn’t turn out the way you want it to.
If someone calls me asking about marking an heirloom handgun from their grandfather, I almost always say don’t do it. You don’t always have a scrap piece of material to test your settings on; if you don’t know what exactly the metal is, then I wouldn’t do it if it’s an expensive item.
If you want to remove the mark, there are really only two ways to do it.
One would be a sandcarving. Sandcarving will take a little bit of the metal surface off any metal, which is what’s required to remove the marks. The marks are essentially bonded permanently to the metal surface. If you can remove the metal surface, then you can remove the mark. Doing this will also change the entire finish of your piece of metal.
The other option can be done using sandpaper. If you sand the metal down, you can usually sand the marks off. It’s a tedious, lengthy process. Normally the times we’ve suggested people do this is for these expensive items, such as an urn. We recommend starting with 300 grit sandpaper, sanding it with the grain of the metal to make those scratch marks on the sandpaper less obvious once you remove the mark. Increase the fineness of the grit over time to bring it back to what it looked like originally.
It’s not something I recommend on any scale, but if push comes to shove and you have something important to you, sandpaper or sandcarving can be the way to go.