I have no experience with the product you mention, but it looks similar to the Elmer's Carpenters Wood Filler (CWF). There are tons of threads if you search on that product. I use CWF a lot and found that I get just as good of results (filling), but have an even easier time sanding if I thin the material a bit with water (doesn't take much) before applying. I like to put a masking tape line on each side of the spiral (within an 1/8" or smaller if you can) and then apply with an old credit card/hotel room key/etc.
Note I've used a version of CWF that was labeled as "Stainable" just like the Plastic Wood X is labeled as. I found that the stainable version had fibers in it that made it much harder to use in the thin layers we need on body tubes. I just put that stuff aside for woodworking use and got the stuff that isn't labeled as "Stainable" and it works much better.