With a few exceptions, welding standards specify both sides, i.e., face and root surfaces, to be finished flush with the surface of the adjacent base metals. This is necessary to ensure the stresses of bending are uniformly distributed and the bend is uniform. The amount removed is just enough to bring the weld flush. Discontinuities such as undercut, incomplete joint penetration, etc. are left intact, i.e. they are not removed. There is no text in the code that states the welded groove can be reduced by an amount equal to the permitted undercut or that incomplete fusion or incomplete joint penetration can be removed to a depth past flush. As a point of interest, the latter is reason to reject the test coupon visually and there is no criteria for porosity or undercut or overlap in ASME Section IX. Like other welding standards, there is no criteria for ugly, only that the specimens pass the guided bend tests or radiographic requirements.

In the case of the 60XX (P23)series aluminum alloys and/or the 40XX (F23) filler metals, the test specimen is machined to 1/8 inch thick. The face bend requires the root surface to be machined and the root bend requires the material to be removed from the face side. 

If one is working to a NAVSEA welding standard; if the joint is welded without the benefit of backing, the root reinforcement is left intact for the root face guided bend test.

Best regards - Al