I have recently started hating Steel-Wool.
Makes a mess by leaving dust.
For my (interior) wood projects, I will used a Foam-Brush to apply Polyurethane and lightly sand with
400 grit paper to remove any bumps followed by a final coat of Poly.
My suggestion: Go to Home Depot and look thru their sandpaper section.
I believe they have various grits with a 'soft' backing pad (about 1 inch think).
This would give you the ability to follow the contour of the spindles.
Enjoy your porch.