One item of furniture started this whole thing off. I'd mentioned to my daughter, Charlotte, that I was looking for an old leather chair, and pretty soon afterwards she rang to say she'd found a second-hand one for sale. It was £30, plus another £10 for delivery. So I bought it. It had original cost over £200, but had been badly scratched by the owner's cat. I thought it sounded like a good restoration project. Here are pictures of the chair in its scratched state.
The first step is to clean up the worst of the scratches. This is a time-consuming procedure, using sharp scissors, scalpel blades, and razor blades to cut back the plucked threads of leather. In extreme cases, an electric hair trimmer can be used to shave the leather back. Then the sanding can begin. Use a medium grade glasspaper to begin with, followed by a finer grade paper. I've used an electric engraving tool, with an abrasive wheel attachment, too. Sand the nibs off the leather, but don't try to level the holes - you will fill these later. The idea is to remove any leather fibres standing proud of the surface.
Next, wipe the surface clean with alcohol - cigarette lighter fuel will also do the job. Leave it for about ten minutes for the alcohol to evaporate. Special leather filler is available for sale, but unless you're going for a professional look, save your money. I use dark brown shoe polish instead. Make a small pad with a clean cotton rag, load it with a little polish, and work it into the leather with small, circular motions. The polish will fill the holes as you work away. Leave the polish to dry, buff with a clean cloth, and repeat over and over again. Think about how you would repair scuffed shoes - you have to repeat the application of polish until the blemish disappears. As you progress, take some warm water, dampen your clean pad, then put some polish onto the pad, and continue with the circular rubbing. Don't be tempted to spit on the cloth - it leaves a streaky surface - always use clean, warm water. The principle is similar to the military practice of 'bulling' your boots. Layers of polish are built up, and buffed back, until a high shine is achieved.
Remember, leather is a 'living' thing. It needs to breathe, and it needs to be fed. Always use wax and oil based products on leather. Lacquers and varnishes suffocate leather. Waxes and oils feed it.