For door grille, the paint shop suggests Hammerite or Nippon Bodelac 9000. This grille is not exposed to the sun or rain, do I really need high-end paints? I have sand down the gate. Does repainting this sort of gate require an undercoat paint?

More worried about my main door. Painting door requires skills. It already has a varnish. Must I totally strip off the varnish before coating the door with a primer, then give it a new coat of paint(probably pastel)? Can I use normal roller instead of the varnish brush?

1. Cost out the price difference between the good paint and the alternative, and then decide if it is worth it to you.
2. Yes and yes