I would recommend to you make your risotto in 3 batches of 10 quarts each.
You could do it all at once, but you loose control over the quality.
This is only my opinion and you might get different answers from other chefs???
No need to make a paste first.
I take my copper pot upside-down and pour a little heap of table salt in the center of it and then wet the salt with a bit of vinegar before spreading this mix with my finger-tips over the whole outside surface of my pot .Rinse well with water and you are done