Stuccano le crepe nell'intonaco prima di verniciare.

Italian often leaves out the subject pronoun when the verb ending already shows who is doing the action.

• Stuccano is third person plural of stuccare
• so it means they fill / they putty / they plaster over

Because of that, Italian does not need loro. The subject is understood from the verb form.

Depending on context, this can mean:

• they in a literal sense
• workers / people in a general sense
• sometimes an unnamed group, similar to English they

Stuccano comes from the infinitive stuccare.

In building or repair contexts, stuccare means:

• to fill with putty
• to patch
• to apply filler
• to seal up small holes or cracks

So here it refers to repairing the cracks before the next step.

Related words:

• stucco = putty, filler, or stucco depending on context
• stuccare una crepa = to fill or patch a crack

Le crepe means the cracks.

Breakdown:

• crepa = crack
• crepe = cracks
• le = the, feminine plural

So le crepe is feminine plural because crepa is a feminine noun.

This also matches the idea that there is more than one crack being repaired.

Nell'intonaco is a contraction of:

• in
  • l'intonaco → nell'intonaco

This happens because Italian often combines simple prepositions with definite articles.

So:

• in + il = nel
• but before a noun beginning with a vowel, il becomes l'
• therefore in + l' = nell'

So nell'intonaco means in the plaster / in the rendering / in the wall plaster, depending on context.

Intonaco is the layer of plaster or render covering a wall.

In English, the best translation depends on context:

• plaster for interior walls
• render for exterior walls
• sometimes just wall plaster

So crepe nell'intonaco means cracks in that plastered surface, not necessarily deep cracks in the wall structure itself.

After prima di followed by a verb, Italian uses the infinitive when the subject is the same as in the main clause.

So:

• Stuccano le crepe... prima di verniciare
• literally: They fill the cracks... before painting

This is a very common structure:

• prima di uscire = before going out
• prima di mangiare = before eating
• prima di partire = before leaving

So prima di + infinitive is normal here.

Yes, the default reading is that the same subject does both actions.

So the people implied by stuccano are also the ones implied in verniciare.

That is why Italian uses prima di + infinitive.

If the subject changed, Italian would normally use a different structure, often prima che + subjunctive, for example:

• Stuccano le crepe prima che il pittore vernici.
• They fill the cracks before the painter paints.

So in your sentence, the most natural interpretation is: the same people patch first, then paint.

Because Italian, like English, can leave the object unstated when it is obvious from context.

Here, after talking about cracks and plaster, it is easy to understand that they are going to paint the wall or surface. So Italian does not need to say it explicitly.

You could expand it if needed:

• prima di verniciare la parete
• prima di verniciare il muro

But in the original sentence, leaving it out sounds natural.

Yes. In practical or instructional contexts, third person plural can sometimes sound a bit like English they meaning people / workers / someone.

So this sentence might describe:

• what workers usually do
• a standard preparation step
• what is being done in a particular situation

Without more context, stuccano can be either:

• a real they
• or a more general, unnamed they

Because the idea is that the cracks are in the plaster layer itself, not simply on its surface.

• nello / nell'intonaco = in the plaster
• sull'intonaco = on the plaster

With cracks, Italian normally sees them as part of the material, so nell'intonaco is the natural choice.

Yes. Stuccano is in the present indicative.

Italian often uses the present tense to describe:

• habitual actions
• instructions
• procedures
• what happens in a sequence

So the sentence can mean something like:

• this is what they do
• this is the normal process
• they are doing this now, as part of the work

The time reference depends on context, but the verb form itself is simply present tense.