The existing slab carries the new surface
An overlay depends on the concrete below it. If the substrate moves, sheds material, holds contamination, or remains damp beyond system tolerance, the new layer can reflect those conditions. A clean-looking corner does not qualify the entire slab. Driveways, patios, walks, steps, and garage floors each have different loads, edges, moisture, and exposure that affect whether resurfacing belongs in the discussion.
Start by mapping cracks, joints, patches, spalls, hollow areas, low spots, and previous coatings. Note places where color changes after rain or snowmelt. Look at edges and transitions, not only the middle. The first fit check should identify disqualifying conditions before time is spent choosing decorative color or texture.