French term (edited): férrer à frais

I've now found a relevant French text, which seems to clarify what this is about. It would, of course, have helped had the Asker provided adequate context. (Why is that failure becoming more and more frequent?)

http://goo.gl/uWffWV
"Quand il est possible de marcher sur cette première couche en chaussette (pour les surfaces au sol) mettre en œuvre une deuxième couche au plâtoir.

"Pour les coloris clairs, afin de limiter le noircissement de la surface au cour [sic] du « ferrage » [sic] éviter l’inox et utiliser de préférence un plâtoir en plastique."

http://www.wikihow.com/Finish-Concrete
"Smooth and compact the surface once again with a hand float. This is the final smoothing, bringing the more consistent "cream" of the concrete to the surface for a level, durable finish."

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Note added at 1 hr (2016-06-24 15:08:02 GMT)
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This was complicated by the fact that férrage usually means fitting hardware to joinery.