Unwanted bevel/underlap issue with Surface Mod

[Furnace]

Hi,
I've built a simple object from some imported splines, added a line to cap top and bottom, used a CrossSection Mod, collapse all then a Surface Mod but I'm getting this unwanted faceting, like a sort of inner bevel (pics attached hopefully), any ideas how to fix would be appreciated, I've tried adding more lines to the spline and and this doesn't help.

Many thanks,
Elliot

P.S. apologies if this post is in the wrong part of the forum.

[svetlozar.draganov]

Would it be possible to attach this scene?

[Joelaff]

Is Fix Dark Glossy Edges turned on in the BRDF section of your material?

[Raven]

What do the beziers on the splines look like? Surface mod respects the curve properties. So if the beziers are really short on the vertical lines your geometry will be bunching up there, which can cause issues with normals and shading.
Can you post a wire frame of the surfaced one?