Hello Gonzalo de Sandoval!
First of all,
The weight of your coin is a bit on the light side but I believe it's still within the legitimate range for this coin, I vote that your coin is a real 8 Reales.
Besides the borderline weight everything else check out including the stamp & edge.
As for the rough surface I have seen it before but I don't know too much about what could have caused it, I have two guesses.
My first guess is that at one time this coin might have had a good amount of advanced verdigris or corrosion and I have read that it can basically eat away at the surface if it gets too bad, judging from the cleaned look of the coin I would guess that maybe it was cleaned to remove that corrosion.
My other guess would be that it might have had something to do with the dies when the coin was minted but I don't feel to strongly about this guess.
Hopefully swamperbob will come onboard soon to tell us what he thinks, he is most knowledgeable when it comes to the Mexican 8 Reales.