Several years ago I used a white, light weight, filler that was sold by 3M. I believe it was sold for repairing drywall or plaster but I found that it also worked with wood. It sanded and "worked" very well.
I know that 3M often drops successful products when they do not meet profit requirements. Perhaps this product was sold and now available from someone else?
Thanks for your suggestions! I am aware of the product you describe. I believe it is gypsum in powder form for mix with water. The drawback to this type of product is grain raise in wood. Thanks again!