This kind of contorted, intentionally incomprehensible Italian (why do they do this?) always requres extensive rewriting before it makes sense in English. One rewrite would be

“Tutte le operazioni di pulitura eseguite in maniera diffusa da strati e/o depositi successivi su materiale lapideo, stucchi, dipinti murari, intonaci e mosaici (con sostante solventi a tampone o a pennello, a secco, ad umido, con impacco ecc.) saranno valutate al metro quadrato o parti di metro quadrato delle porzioni di materiale interessate”

Here's what I've come up with after a few minutes' work:

All general cleaning with dry or wet solvents applied in successive layers and/or deposits (with a pad, a brush, a pack etc.) on stone, stucco, wall paintings, plasterwork and mosaics must be assessed in terms of the square metres or parts of a square metre of the portions of material concerned.


... but still more work is needed to this. And good luck with the rest of your text! If it's all like this bit, it's going to require a lot of hard work...

Hi Anthony,

just another option .........

http://jacopo-russo.it/MAE_definitivi_alta_risoluzione/A12_O...
search terms:
soprammessi=laid over

HIH Grace