Just a quick question, is it normal for there to be a gap along the surface of a mesh once the screw modifier has been applied?
There can be gaps but they're not inevitable.
- The Angle should be 360° so the edges line up.
- The Screw value should be exactly the same value as the object's dimension in the relevant axis.
- The highest and the lowest vert should have the same distance from the screw axis.
- After applying the modifier, using Remove Doubles with the relevant verts selected and a large enough threshold should kill any small gaps. Without that, a Subsurf Modifier might produce a gap.