Learn French: Il ne veut poncer que le bois massif. - He only wants to sand the solid wood.

In the ne... que (only) construction, the que must go immediately before the specific thing being restricted. He doesn't 'only want' something; he wants to sand 'only the solid wood'. If you wrote Il ne veut que poncer le bois massif, it would slightly change the meaning to 'He only wants to sand the solid wood' (implying he refuses to paint it or cut it).

Why does the ne go before veut and not before poncer?

Even though the restriction ultimately applies to the object of the infinitive (poncer), the ne in this structure must always precede the main conjugated verb in the clause. Therefore, it goes before the conjugated verb veut.

Does massif mean 'massive' or 'huge' here?

No, this is a slight false friend! When talking about materials like wood or metal, massif means 'solid' (as opposed to hollow, veneered, or plywood). Le bois massif is the standard French term for 'solid wood'.

Could I use seulement instead of the ne... que structure?

Yes, you could completely drop the ne... que and say Il veut poncer seulement le bois massif. However, ne... que is extremely common in both spoken and written French, so mastering its word order across dual verbs is essential at the B2 level.