Honestly, the trick is to stop grit and stop sliding. In a city balcony, dust, grit, and small bits of debris blow in constantly, and they act like sandpaper under furniture legs. So before I worry about fancy solutions, I sweep or vacuum the decking regularly—especially after windy days—and I check under chairs and planters for trapped grit.
For the furniture itself, put protective pads under every leg. Felt pads work well for lightweight chairs and tables, while rubber or silicone caps are better for heavier pieces that tend to shift. Check them every few weeks, because worn pads can collect dirt and become scratchy themselves. I also like small coaster-style cups under planters and furniture feet; they spread the weight and keep hard edges off the decking.
If you move furniture often, lift it instead of dragging it. I know it is tempting to slide a chair over a few inches, but that is exactly how fine scratches appear. For heavy items, use furniture sliders only while moving them, then remove the sliders so they do not trap grit.
An outdoor rug or interlocking deck tiles can also create a protective buffer in the busiest area. Just make sure water can drain and the underside does not stay damp. And avoid black rubber feet on composite decking, since some can leave marks over time—choose non-marking pads instead.
Finally, keep the layout simple and leave a little breathing room between furniture and railings. The less you have to push things around, the less chance there is of scratching. Simple, cheap, and renter-friendly—but it really works.