Border's backs ran in tight formation on angled raids out wide – opening up plenty of gaps as they streaked onto long passes from halfback Lindsay Horrocks, who played the first half after backing up from his Hurricanes Development XV game on Thursday.
No8 Angus Middleton was unstoppable early, scoring two smart tries with a fine turn of pace.
Despite the wet conditions, Harding brought his kicking boots, landing eight conversions from nine attempts in a 21-point haul.
Ratana brought on some bigger but slower forwards for the second stanza, including veteran Isaac Fonotoe, who introduced a bit of mongrel.
While they then held Border out from long patches, they still played all their rugby inside their 30m zone.
Home prop Tyson Tauri defended well in the first half, while winger Dale Akuhata showed he has some pace but not yet the execution, with Ratana blighting most of their rare chances.
Border's other standouts were prop Kamipeli Latu making tough yards, while lock Jack Hodges pretty much ruled the lineout no matter who was throwing it in.
"Ruapehu [beating us] probably helped us a bit, in relation to being able to reflect," said Border coach Ross Williams.
"Today was about playing to our structure.
"We unlocked the space, and that's a credit to our boys.
"Given the rain, happy to keep them territorially out of our end."
Ratana were without regular coach Steelie Koro due to a family bereavement, and his assistant Johno Kaa was just disappointed their team could not put some points on the board.
"We've always had a better second half this year, but I don't know what it is.
"We try to do the Sonny Bill passes, when we should just plug away. Rushing too much.
"Everyone comes to training, everyone still cares. We're still in it, we'll never default."
Ratana tried to kick for territory early, and after making some mistakes, Border clicked into gear as Horrocks shifted wide and found winger Tom Matthews, who ran to a half gap and offloaded to Harding, who in turn sent Middleton on a long dash to the line.
Middleton needed no help on his second try as he stepped through the Ratana backs and then wrong-footed fullback Viliame Kuruyabaki to go under the posts.
Winger Isaiah Hooper held up to wait for a chip kick, but was still able to beat the approaching cover defence and to dive over beside the posts.
Harding ran wide on another long pass and offloaded to Tuimauga, whose loose ball to second-five Opetini Dryden was claimed with an outstretched arm and he ran on to score.
Border's backs kept sweeping to the far left of the field and this time Harding was on the end of the chain to beat his man and cross at the corner flag, before landing his first difficult conversion attempt.
The visitors kept it up after halftime, as any lapses they made were quickly recovered by stealing lineouts or blowing over rucks, with first-five Tyler Rogers-Holden running onto a long pass and gliding through with a fend to dash under the bar.
Having missed out so far, Symes had a crack with his extra yard of pace and was able to slide in with the tackler on his back for 49-0.
However, from here there would be a long stretch without points as Ratana gamely tried to use their bigger reserves for impetus, but were still playing all their rugby in sight of their posts – running it out but turning the ball over and having to defend grimly.
Border subbed key players like Horrocks and Middleton, and the previously efficient formations looked a little less steady, with players on both sides starting to cramp up in the rain.
A Hodges hit-up finally settled Border at the tryline, with prop Hamish Mellow being pushed over the chalk.
Ratana halfback Cornel Mason tried to spark something with his loose forwards from a 5m scrum, but it fell apart and Rogers-Holden couldn't believe his luck as he snatched the ball to score.
Ratana were desperate for any points and had a chance from the restart, but Border broke out and Matthews chip-kicked and then kept the ball on his leg, before Symes deftly snatched it up and ran to the posts — drop-kicking the extras on fulltime.
Border 68 (A Middleton 2, T Rogers-Holden 2, H Symes 2, I Hooper, O Dryden, N Harding, H Mellow tries; Harding 8 con, Symes con) bt Ratana 0. HT: 35-0.
