I have used a very sharp scraper for this purpose in the past. A Skarsten hook scraper can, when used with care, remove fine shavings of paint/varnish and wood. Use with extreme care on ply as you can easily go through the top veneer. This technique can be very effective with practice and works well into corners that are difficult to sand. Sharpen the scaper with a fine sharp file or on a stone and keep it sharp, it is a dull tool that causes damage.