That's pretty wide coaming. If you step on that, I would not want varnish on it. How are you treating after you're done? I've gone old school, let it weather and grey, other than a few accent pieces. If you go grey, just let the varnish flake off over time. Time = couple of years.
To your question, sanding is your best bet. Personally, I'm a random orbital guy. Belt sanders require experience and can get you in trouble fast. Rent an RO if you don't have one.