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?