Prima di verniciare, levighiamo e stucchiamo l'intonaco.

Because prima di is normally followed by an infinitive when Italian means before doing something.

So:

• prima di verniciare = before painting / before we paint

A finite verb like verniciamo cannot follow di here.

If you want a full clause with its own subject and verb, Italian usually uses prima che + subjunctive instead:

• Prima di verniciare, prepariamo la superficie.
• Prima che Mario vernici, prepariamo la superficie.

Use prima di + infinitive when the subject is the same, generic, or not important.

Use prima che + subjunctive when you want to express a full clause with its own subject.

For example:

• Prima di verniciare, levighiamo l'intonaco.
  = Before painting, we sand the plaster.

• Prima che loro vernicieno, levighiamo l'intonaco.
  = Before they paint, we sand the plaster.

So in your sentence, prima di verniciare is the normal compact structure.

They are first-person plural forms, meaning we.

More specifically, they are the present indicative forms of:

• levigare → levighiamo
• stuccare → stucchiamo

So literally, the sentence says we sand/smooth and fill the plaster before painting.

In practical or instructional language, Italian often uses noi forms like this where English might also use an instructional we.

Yes, in the right context it can.

The noi form can function as:

• a normal statement: we sand and fill
• an inclusive instruction: let’s sand and fill

In a manual, tutorial, or spoken instruction, the line could sound like an instruction to a group. Context decides.

Because Italian usually drops subject pronouns when the verb ending already makes the subject clear.

The ending -iamo already tells you the subject is we.

So:

• levighiamo = we sand/smooth
• stucchiamo = we fill

You would add noi only for emphasis or contrast:

• Noi levighiamo e stucchiamo l'intonaco, loro verniciano.

This is a spelling rule used to keep the hard sound of g and c before i.

• levigare → levighiamo
  The gh keeps the g hard, like the g in go.

• stuccare → stucchiamo
  The ch keeps the c hard, like the c in cat.

Without the h, the pronunciation would change.

Because intonaco starts with a vowel, and the singular definite article is shortened before vowels.

Here the full article is il, which becomes l' before a vowel:

• il muro
• l'intonaco

So l'intonaco simply means the plaster.

Italian uses the definite article more often than English.

Here l'intonaco refers to the specific plaster surface being worked on, so the article is natural and expected.

Italian often says:

• levigare il legno
• pulire il vetro
• stuccare l'intonaco

where English may sometimes omit the depending on style.

Because it is being treated as a material or surface: the plaster/render as one whole thing.

Italian often uses the singular for materials or surfaces in this kind of practical sentence.

If you were talking about several separate plaster finishes or layers, a plural might be possible, but in this sentence the singular is the normal choice.

Yes. L'intonaco is the direct object of both levighiamo and stucchiamo.

So the structure is:

• levighiamo l'intonaco
• e stucchiamo l'intonaco

Italian, like English, usually mentions the object only once when two verbs share it.

Not strictly always, but it is very common and natural.

Prima di verniciare is an introductory phrase, and the comma helps mark a pause:

• Prima di verniciare, levighiamo e stucchiamo l'intonaco.

In shorter, less formal writing, you may also see it without the comma, but with the comma the sentence is clearer and more polished.

Yes. Italian word order is fairly flexible.

For example, you could also say:

• Levighiamo e stucchiamo l'intonaco prima di verniciare.

That means essentially the same thing.

The original version puts prima di verniciare first, which emphasizes the sequence: before painting, this preparation happens first.

Yes. Verniciare usually means to coat with paint/varnish, especially in a practical, surface-treatment sense.

It is different from:

• dipingere = to paint more generally, including artistic painting
• tinteggiare = to paint walls / whitewash / apply wall paint

So verniciare gives the sentence a practical, finishing-work feel.