When you're repainting your house, here's how to restore that smooth, original finish. After you've removed all loose paint and sealed the surface with a latex primer, use a water-based exterior-grade spackle to fill in the rough areas that remain. With a four-inch putty knife, spread an even layer of spackle and smooth it out. When it's dry, lightly sand it and apply a second coat the same way, if necessary. When the surface is perfectly smooth, apply your finish coat.
For more on painting, consider:
Exterior Paint 101
Radio: Exterior Painting Prep
Exterior Painting Preparation (VIDEO)