Back to front with the coats. More coats you apply, the thiner they become, as you are applying "gloss", finish and not "material", polish.
Polyurethane is notorious for pin holes, air bubbles trapped in the coating.
You need to use a good thick pigs hair brush with a straight end. It is the flowing of the coat that traps the air bubbles so work slowly in one direction and don't overbrush, that is, continue brushing over the applied coat.
(Applying "polish" is different to painting. When painting you work back into the applied coat to eliminate brush marks. However 'Plastic' paints have virtually eliminated brush marks.)
Subsequent coats, after the base coat/layer should be thinned, and lightly sanding and cleaned off between coats.
Experiment on a little scrap, to get the technique right, as you will see the bubbles occurring, if you are not doing it right.
These days you can get equally as good a finish with other simpler produsts instead of using Polyurethanes.
Those locally can advise what is available and what they use.