What grade of textile should I use to ensure a good surface finish?

A smooth textile with little crimp tends to give a smoother surface finish. Something like a UD or lightweight fabric with a twill or satin weave. Avoid a heavyweight fabric with a lot of crimp such as a plain weave as this can lead to a bumpy surface. Surface finish is also dependant on the method used to create your panel e.g. compression moulding/ autoclave/ resin infusion etc and the care taken to clean and prepare the surfaces prior to layup.