Thanks brandfbr.
The idea of the oil based first coats is to not raise the grain of the timber (like water-based coats would). I have been spraying water-based poly final coats on my last couple of guitar builds but found that the grain still raised a bit. So, this is another trial at trying to reduce raised grain.
Note: I also use a Ubeaut sanding sealer (shellac based) before the stain to raise the grain a bit (and it can be lightly sanded (de-nibbed)). I have also used Timbermate grain filler (water based) to grain fill and raise the grain. Others also wipe the timber with a damp (water or metho) cloth to help lift the grain.
This is probably only an issue when using a stain, on open grained timber. After staining, you do not want water-based coats raising the grain of the wood and making it rough, since you cannot sand it back too much as you may remove the stain layer.
For non-stained timber it is not a problem as you can sand the raised grain back and then add more water-based coats.
Seeing fender3x's work, I have thought that Shellac may be a better alternative to the oil-based poly first coats!