Mix a small amount of varnish with oil-based or alcohol stains, rub directly into the crack or scratch, and rub until it gets sticky, then rub a little more to "cook" it. Continue this the next day, perhaps with a slightly darker or lighter stain. Other things that
work include a drop of iodine for red, a quick swipe with a fine tip black marks-a-lot, or even shoe polish. Keep recoating at intervals to fill the scratch gradually. This is basically what French polishing is all about. Dirty little secrets from the
trade.