Spanish to English Art/Literary Metallurgy / Casting

Hi,

I'm subtitling a short documentary in which the narrator talks about how the legs on a wrough iron table were painted. I've come unstuck with the following term:

"Después de tres capas de pintura negra, se aplica otra capa de rugosidad y una antioxidante."

I'm not exactly sure what the purpose of the "capa de rugosidad" is or what the English equivalent would be.
Any input would be appreciated!

Thanks in advance.

Dez