Astar boya sökücüyle yumuşadı.

Here astar means primer / undercoat, the layer put on before the main paint.

A learner may know astar from other contexts too, because it can also mean lining (for example in clothing). In this sentence, because of boya sökücüyle (with paint remover), the painting meaning is the relevant one.

Boya sökücü literally means paint remover or paint stripper.

It is built from:

• boya = paint
• sökücü = remover / stripper

Sökücü comes from the verb sökmek, which can mean to remove, to strip off, to detach, depending on context.

So boya sökücü is a very natural Turkish way to say paint remover: literally, paint-removing substance/tool.

-yle here means with or by means of.

So:

• sökücüyle = with the remover / using the remover

This ending is the attached form of ile:

• boya sökücü ile
• boya sökücüyle

Both mean the same thing, but the attached form is very common in everyday Turkish.

Because the sentence is talking about one paint remover / paint-removing substance in a general singular sense.

The form is:

• sökücü
  • ile
• because sökücü ends in a vowel, Turkish inserts a buffer y
• so it becomes sökücüyle

Also, the vowel in the suffix is e, not a, because of vowel harmony: the last vowel in sökücü is ü, which is a front vowel, so the suffix appears as -le / -yle.

Yumuşadı is the 3rd person singular past tense of yumuşamak.

Breakdown:

• yumuşa- = soften / become soft
• -dı = past tense

So yumuşadı means:

• it became soft
• it softened

Here the subject is astar, so the meaning is the primer softened.

This is an important distinction.

• yumuşamak = to become soft / to soften
  
  • intransitive
• yumuşatmak = to soften something
  
  • transitive / causative

So:

• Astar boya sökücüyle yumuşadı.
  = The primer softened with paint remover.
• Boya sökücü astarı yumuşattı.
  = The paint remover softened the primer.

In your sentence, astar is the thing undergoing the change, so Turkish uses the intransitive verb yumuşadı.

Because astar is the subject of the sentence, so it is in the nominative form, which usually has no visible ending in Turkish.

So:

• astar = the primer (subject)
• boya sökücüyle = with paint remover
• yumuşadı = softened

Turkish often leaves the subject in its plain dictionary form unless another case is needed.

The sentence follows a very normal Turkish pattern:

• Astar = subject
• boya sökücüyle = instrumental phrase
• yumuşadı = verb

So the structure is:

Subject + with/by-phrase + verb

Turkish usually places the verb at the end, so yumuşadı naturally comes last.

Here it means using / by means of, not together with.

That is, the sentence means the primer softened because of / through the use of paint remover.

Turkish ile / -yle can express both:

• together with
• by means of / with

You understand which one is meant from context. In this sentence, the only natural reading is the instrumental one: with paint remover.

It is understandable and grammatically fine. But Turkish speakers might also say the idea in a more direct way, depending on what they want to emphasize.

For example:

• Boya sökücü astarı yumuşattı.
  = The paint remover softened the primer.

This version makes boya sökücü the grammatical subject and uses the transitive verb yumuşattı.

Your original sentence focuses more on what happened to the primer, while this alternative focuses more on what the remover did.

Yes. You can write:

• boya sökücü ile
• boya sökücüyle

Both are correct.

In modern Turkish, attaching ile to the previous word is very common, especially in ordinary prose and speech-based writing. So sökücüyle is completely normal.

A very literal translation would be something like:

• The primer softened with paint remover.

That is close to the Turkish structure, but in natural English people might often prefer:

• The primer softened after paint remover was applied.
• The paint remover softened the primer.
• The primer became soft with the paint remover.

So the Turkish sentence is straightforward, but the best English translation may depend on style and context.