Thanks to Taña for the references.

So encascar means "Teñir o dar casca a las artes y aparejos de pesca"
http://dle.rae.es/?id=F2DD7sQ
And casca is "Hollejo de la uva después de pisada y exprimida."
http://dle.rae.es/?id=7mVx6ek
In other words, dying fishing nets with pomace.

An "encascador" is the place where it's done. This is from a book called Género, trabajo y niveles de vida en la industria conservera de Galicia:

"En él [el patio] se situaba la caldera de bronce y hierro para preparar el tinte de las redes y una poza para tintar varias a la vez. En el encascador había espacio para las principales materias usadas: corteza de pino, alquitrán, brea, pinturas, etc."
https://books.google.es/books?id=T6udwMpylCIC&pg=PA40&lpg=PA...

This is interesting, because it shows that they didn't only use pomace in the "encascador" but also bark and pitch.

Well, here's a passage from a book published in 1837 called Notes on nets:

"Of Dying Nets.
[...]
A Russet colour is produced by tanning, which has likewise the advantage of preserving the net (vide suprà "Tanning"), or by immersing the net in a decoction of logwood."
https://books.google.es/books?id=ZEUCAAAAQAAJ&pg=PA58&lpg=PA...

I think we're clearly talking about the same operation, and the verb to use for "encascar" is "tan". This verb commonly applies to leather, but not only: nets are (or were) tanned too, as we can see. And I'm sure the Galician fisherman were not just trying to make their nets a pretty colour but to toughen and preserve them too.

The place were you tan is called a tannery. And here's a painting of a net tannery in Cornwall, a region pretty comparable to Galicia:
https://picclick.co.uk/Tom-ONeill-1946-Watercolour-Old-Net-T...

As usual with these words for local customs, it's a good idea to keep the Galician word and add a translation. For "tinte de redes (escascador)" I would put "net tannery (escascador)".

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Note added at 1 hr (2018-10-08 19:09:13 GMT)
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I've just noticed that my Spanish reference is the same as Taña's, though from a different source.

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Note added at 15 hrs (2018-10-09 09:14:46 GMT)
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Sorry: dyeing, not dying (hopefully).