The guitar build nearing the point where I need to think about the finish of the guitar, and having used Sepele, Black Walnut and Oak I felt it would be a shame not to use a transparent lacquer. However a test spray quickly showed how open grained all three are. The largest grain or pore voids by far are in the oak. I looked at all sorts of grain filler and followed a lot of posts suggesting cyanoacrylate (CY, crazy glue or superglue) which looked promising.
However in attempt to speed this up I decided to use a gold metallic acrylic paint to fill the large gaps and then CY glue to then grin fill the rest. I added about 30% volume of water to the acrylic paint and then rubbed it into the surface removing as much excess from the surface as possible. I then rubbed it down with 400 grit and then sealed with superglue and left it to set. After initial sanding the finish is better than I expected. The gold has filled the largest voids and is _just_ visible when at the right angle. The end grain took more paint but here is an image of the top. I am very happy with this grain fill, the gold goes really well with the oak.
I will add that I masked the black walnut and Sapele off so they just had the superglue grain fill. The superglue took three wipes and sanding to fill all the grain, but it's easy to work so it's just time spent more than anything else.
Below, poor picture but you can just make out threads of metalic gold acrylic grain fill in the oak. It looks slightly proud of the surface, but isn't.