I don’t know if the kiln wash will work. I do know that you can buy Satin Cast (original) which will give you a very smooth finish. It’s made for metal casting, but it works well with glass.
Mixing your own is certainly much cheaper and I have had good results from home-made mixtures. The finish is not as smooth as Satin Cast, but I have been able to smooth out the interior with fingers, cloth, cotton swabs, and the makeup sponges available in the cosmetic aisles.
When that fails, fire polishing helps. The “tooth” that the homemade investments leave is something which can add to the final item. It really is a matter of taste and application.