1)
What is the significance of asking "where is the plaster"?
1.
Rashi: When the evil will come, he will say to you 'where is the Shalom that you promised'?
2.
Malbim: Since the wall will fall, all the more so the plaster will!
2)
Why will he ask about the plaster, but not about the wall?
1.
Radak #1: They will ask also 'where is the building that you built'? The verse mentions the last matter mentioned, for brevity.
2.
Radak #2: Those who build the wall will not be [around] after it falls. The false Nevi'im will be eradicated. Some plasterers will remain; he will ask them about what they did. "Kir" represents the wall of Yerushalayim, which was breached.