We have a 1920's bungalow with lath and plaster ceiliings. The ceiling was cracked and i have removed the plaster; the lath remains to hold up the blow-in attic insulation. Installing sheetrock directly over the lath seems problematic since there is a small residual amount of plaster that creates a slightly uneven surface. Any ideas on what we should do to create a nice flat surface for the sheetrock? Thanks!