I don’t have any personal experience washing lampshades—you can’t run them through a clothes washer, unfortunately, so I stick to keeping them vacuumed. But If I had one with water spot on it, I would want to experiment with getting it out, so here’s what I would consider before trying anything.
–How is the fabric attached to the frame? If it is glued on, then if you submerge the lampshade in a detergent and water solution (or even just water) the lampshade cover will fall apart as the glue dissolves, so it may be time to recover the lampshade instead of trying to clean it. If the fabric is sewn on to the frame, then you may be able to wash it.
–Do you know anything about the type of fabric that covers the shade? If it’s silk, silk can end up with water spots once it is dry, so that’s not an easy fabric to try to hand wash. If you already tried to wash a stain off the lampshade with a little water and that’s how you got the water spot in the first place, then you may be able to unify the appearance of the fabric by getting it entirely wet, but there’s no guarantee of success. If the fabric is cotton, polyester, nylon, or acrylic then you have a better chance of successfully washing it.
–Is there a local professional that can help you with a recommendation for a cleaning service? Ask at the local lampshade shop, if you have a good one. I would definitely do this if the lamp is a family heirloom.
If you decide you want to try washing the lampshade, ideally it is small enough in size to fit in a 5 gallon bucket; otherwise you may need to work in the bathtub. If the lampshade is pretty new and clean except for the water spot, just submerge the entire lampshade in fresh water in the bucket until it is completely wet. Remove it from the water, and set it on a clean white towel. Keep it out of the sun while it air dries. If the lampshade is older and a little dirty, then try adding a little mild detergent to the water. After you have submerged the lampshade in the detergent and water solution and swirled it around a little, empty the solution and refill the bucket with clean warm water. Submerge the lampshade again and swirl it around, and then empty the bucket. Repeat this as many times as you need to until any remaining suds are rinsed away. Then you can set it on the clean towel to air dry.
Unfortunately there’s no guarantee that any of this will work. There are lots of videos on-line with people washing lampshades, but they don’t show the successful finished clean lampshade, only the process. That’s probably because lampshades are inherently difficult to wash with water. Fortunately they aren’t very expensive to recover; this makes a good DIY project if you can find new fabric you like.
I wish I could have been more helpful with a definitive “this will work answer”. Please let me know how it goes, and thanks for writing.