Το υάλωμα ετοιμάστηκε, μόνο που η εφυάλωση αρχίζει αύριο.

They are related, but they do not focus on exactly the same idea.

• υάλωμα usually refers to the glaze / glazed layer / glazing material / finished coating
• εφυάλωση refers more to the process of glazing

A very common pattern in Greek is:

• nouns in -μα often refer to the result of an action
• nouns in -ωση often refer to the process or action

So the sentence is contrasting:

• the glaze or glazed preparation being ready with
• the actual glazing process starting tomorrow

Because the two nouns have different grammatical genders.

• υάλωμα is neuter, so it takes το
• εφυάλωση is feminine, so it takes η

This is very normal in Greek:

• nouns ending in -μα are usually neuter
• nouns ending in -ωση are usually feminine

So the articles are simply agreeing with the gender of each noun.

Greek uses the definite article much more often than English.

In normal Greek prose, when the speaker is referring to a specific thing already understood in the context, the article is very natural:

• Το υάλωμα
• η εφυάλωση

Even where English might omit the, Greek often keeps it. So this is not unusual or overly formal; it is standard Greek usage.

ετοιμάστηκε is the 3rd person singular aorist passive of ετοιμάζω (to prepare).

So grammatically it means:

• was prepared
• or sometimes more naturally, got prepared

Breakdown:

• verb: ετοιμάζω
• aorist passive stem: ετοιμάστ-
• ending: -ηκε

In this sentence, it gives a completed action in the past: the preparation has already been done.

Greek often uses the present tense for a scheduled or expected future event, just like English does.

Compare English:

• The meeting starts tomorrow.

That is exactly the same kind of idea here:

• η εφυάλωση αρχίζει αύριο

So this does mean a future event, even though the verb is in the present.

A more explicit future would be:

• η εφυάλωση θα αρχίσει αύριο

Both are possible, but αρχίζει αύριο sounds very natural for something planned or scheduled.

Here μόνο που means something like:

• except that
• only that
• it’s just that
• the only catch is that

So the sentence has this structure:

• one thing is already ready
• but there is one limitation: the actual process starts tomorrow

So μόνο που is not just a literal only here. It introduces a qualification or small problem after the first statement.

Yes. Greek word order is fairly flexible.

Both of these are possible:

• Το υάλωμα ετοιμάστηκε
• Ετοιμάστηκε το υάλωμα

The difference is mostly about focus or style, not basic meaning.

• Το υάλωμα ετοιμάστηκε puts the glaze first as the topic
• Ετοιμάστηκε το υάλωμα puts a little more attention on the fact that it was prepared

So the original sentence sounds natural, and the noun-first order helps set up the contrast with the second clause.

Not really. They sound specialized or technical.

You are more likely to meet them in contexts like:

• ceramics
• materials
• coatings
• industrial processing
• art or craft terminology

Especially εφυάλωση feels quite technical or formal. So this sentence is grammatically normal Greek, but the vocabulary is not especially everyday conversational vocabulary.

They are similar, but not identical.

• αλλά is the basic word for but
• μόνο που often means but the thing is that... or except that...

So μόνο που usually adds the feeling of a qualification, a small obstacle, or one important detail.

In this sentence, μόνο που works well because the first part sounds positive:

• Το υάλωμα ετοιμάστηκε

Then comes the limitation:

• μόνο που η εφυάλωση αρχίζει αύριο

So αλλά would be possible in some contexts, but μόνο που gives a more specific nuance here.