Hi Marlin,
first of all welcome.
The black marks which can surface while burnishing are normally due to the use of a not-inox steel trowel.
Second frequent cause is the overburnishing i.e. burnishing the same square foot too long.
In both cases sandpaper will not help preventing the marks.
Should you have the black marks already and should you wish to remove them, then you have to remove (at least partially) the finish. In that case sandpaper is obviously useful.

That might be a good Ideal Plastic trowel especially on the light colors. I burnish columns and curved walls with plastic bondo scrappers. I even burnish with my fingers or a cloth when needed.

Inox is made for gritty plasters.

I think plastic would work just like we use it on the Firoentino.

What do you think Luca, use of the plastic trowel on the really light colored Veneziano's.

Vallie

Sorry Vallie, I had missed this post.
We tried a couple of plastic trowels with Veneziano and were unhappy. Both were quite "soft" and kept on denting on the "blade". The final results were unsatisfacory but there's no specific chemical reason why a plastic spatula would not work. Maybe a harder plastic trowel would do but we didn't find any yet....
Luca