Flexible surface types on irregular grids, such as T-splines and LR-splines, are gaining popularity in science and industry due to the possibility for local grid refinement. We present a novel rendering algorithm for those surface types that guarantees pixel-accurate geometry and water-tight tessellation (no drop-outs). Before rendering, we extract the Bézier coefficients. The resulting irregular grids of Bézier patches are then rendered using a multistage algorithm, that decouples the tesselator and the patch geometry. The implementation using OpenGL utilizes compute shaders and hardware tessellation functionality. We showcase interactive rendering achieved by our approach on three representative use cases.