The Welsh visitors had added 83 tailender runs on Friday morning, as Chris Cooke ended on 136, before sharing the wickets around to leave Northants slumped on 76 for five.
It is the third Championship innings in a row that the County have suffered an alarming batting collapse.
But skipper Rossington and Saif Zaib put on 126 for the sixth wicket, with the former notching his first red-ball half-century since last August.
Their efforts, along with an unbeatne 25 from Wayne Parnell, meant Northants steered their way to 251 for seven, six runs away from avoiding the follow-on and within 156 runs of Glamorgan
Resuming on 324 for seven, Glamorgan enjoyed almost the perfect morning as they ticked off a fourth batting point, rose above 400 before taking two early Northants scalps.
Partnerships of 46, 26 and 32 for the last three wickets frustrated the home side – under pure blue and above a slowish wicket.
James Harris was watchful for his 17 before he was caught behind, but James Weighell and Michael Hogan scored in extravagant style all alongside the unflappable Cooke.
Weighell, who got a pair on county debut last week, struck Sanderson for a six before edging Wayne Parnell behind, while Hogan thrashed six boundaries in his unbeaten 25.
Cooke finally exited after an excellent innings, caught at first slip when attempting an offside slog, as Glamorgan ended up with 407.
Pick of the Northants bowlers was Gareth Berg, who ended with three for 69 from 25 overs, while Parnell claimed three for 82 from 20.
Their profitable morning continued when Ricardo Vasconcelos edged a cut shot to Billy Root at first slip.
Weighell ended Northants’ miserable session when he ran out Charlie Thurston with a sensational direct hit from midwicket.
Rob Keogh, Luke Procter and Ben Curran all fell soon after lunch to leave the hosts in a mire.
Keogh was lbw to Harris, who took his first Glamorgan wicket since dismissing Monty Panesar 2,441 days ago having returned to his boyhood county on a two-week loan from Middlesex.
Curran, having reached 1,000 first-class runs for Northants on his 21st appearance, was caught behind off Weighell before Procter gloved a ball down the legside to keeper Cooke five balls later.
Rossington arrived and countered, typified by two sixes in a Callum Taylor over – firstly over long-off and then clipped to the short square boundary – while Zaib blocked from the other end.
Having reached 40 in a brisk 48 balls, it took Rossington a further 24 deliveries to negotiate his way to a half-century, brought up with a nicely timed square drive.
The duo both departed within 11 deliveries of each other as Zaib swung a David Lloyd knee-high full-toss straight to mid-on and Rossington was pinned by Michael Hogan.
Parnell, who was dropped at second slip on four, and Gareth Berg guided their side to the close without further damage.
LV= Insurance County Championship
Northamptonshire vs Glamorgan at Wantage Road, day two
Glamorgan 407 all out, lead Northants 251-7 by 156 runs