If you're looking for remoteness in the Cascades, then Primus is primo. I have often wondered what peaks in the Cascades are the farthest from any designated trail or road. Certainly, there are those in the Pasayten Wilderness that might be supposed to be remote, but it seems as if each and every valley in the Pasayten has a trail in it, meaning any mountain on an intervening ridge will be close to said trail. So one must look elsewhere. Primus has got to be one of these peaks. Another would be nearby Austera Peak. In fact, Austera is farther from a trail than Primus.
As a mountaineering challenge, Primus' value is in the arduousness of its approach routes, of which there are two. The climbing itself on the mountain massif is largely trivial (though steeper, unaesthetic routes are available). From the south and west, the peak is a walk up (Class 2). On the east it is Class 3. I don't know the difficulty of the north side. A technical climb was undertaken on the North Ridge by Mark Bebie in 1986 but I imagine this is very seldom done.
As said, there are two approaches to this peak. Any other approaches are up to you and your sense of adventure. Not that the ones I will present here aren't full of their own adventure. The first approach and probably the most used is via the Eldorado ice cap. This is a multi-day outing. Two days is possible, but if you're going to be in the area you might as well stay an extra day to enjoy the other peaks there (Tricouni, Austera, and Klawatti peaks, also Eldorado Peak, Dorado Needle, and innumerable technical crags). The second approach has been described as hellish but doable. This is the approach from the north.
Approach 1 (Eldorado Ice Cap)
This is the same as the standard approach to Eldorado Peak. From Marblemount on the North Cascades Highway (SR-20), drive 19 miles east on the Cascade River Road. A parking lot on the right with a toilet is your trailhead (~2,150 ft). The first order of business is getting across the river to its north side. There used to be a big log a few hundred yards downstream of the parking lot but it has since been washed away (probably by the rainstorm of 10/20/03). Our early-April 2004 trip to Primus used a crossing about 100 yards downstream of the parking lot. This crossing used three logs and three gravel bars. I'm not going to present detailed info for the crossing as it seems to change all the time. Suffice it to say, once you get across, the second order of business is to locate the climber's trail on the other side. This is not an official trail, hence the park service doesn't maintain it at all, which is too bad (they should given the amount of traffic it sees). The trail is maybe 140 feet into the forest on the other side and sort of parallels the river for a stretch (40 yards maybe) before turning uphill. The trail is west of Eldorado Creek but not too far west. It seems a lot of people can't find the trail on the other side of the river and crash brush upwards too far west of the creek. The best thing to do is take the extra time to locate the trail down in the flat before commencing uphill. The trail is actually quite good considering it's not maintained. However, it is steep and rooted (and windfallen--especially down low), so care must be taken.
For 1,800 feet, the trail steadily climbs up through verdant forest and ends at ~4,000 ft on the left side of "the boulderfield from Hades." In early season (on or before April), this boulderfield is snowcovered. Backcountry skiers can float right over the top of them. However, as melting goes on, more and more boulders and their associated interstitial pocket pitfalls lurk to ensnare the unwary skier. Not that it would be any easier for those booting it through. If the snow is unconsolidated or if it is warm, this can be hell on earth.
The boulderfield relents eventually. In summer, a trail forms on its eastern fringe (but still west of the creek). The trail turns into a path and finally opens up to the alpine terrain of Eldorado Basin at 5,600 ft. A lot of parties elect to camp in this basin. Others continue on for a camp closer to Eldorado Peak. This requires one to climb left over a rib into the next basin west (Roush Basin). I have always thought the geographical naming strange in that Eldorado creek and basin don't actually drain Eldorado Glacier, where as Roush Basin (and Roush Creek) is the one that drains the glacier. Seems like the names are backward. Anyway, there is a flat section on the rib at ~6,100 feet that allows access to Roush Basin. The west side of this rib can be tricky in summer--especially for beginners. Slabby rock and heather with some exposure if you fall make for an interesting downward traverse for a few hundred feet. In winter this is all snowcovered and much easier (ice axe might be handy--especially if icy/hard snow in the wee morning hours). Once into Roush Basin, simply climb up the eastern fringe of the Eldorado Glacier for about 1,300 ft to the large snow and ice dome (c. 7,500 ft) immediately SSE of Eldorado Peak. Gawk at the beauty before and behind you. At this point you're still a good five miles and four basins away from Primus Peak. From a camp here it would be possible to climb Primus and return to camp within a day. Alternatively, you can proceed further to Klub Klawatti at the 7,800-ft col on the west side of Klawatti Peak.
To get to Klub Klawatti Kamp, cross north from the snow and ice dome to the small notch/saddle at ~7,500 ft on the lower East Ridge of Eldorado. Continue beyond this for 1.5 miles to the Klawatti Col. Depending on season, crevasse routefinding will be more or less.
Climb 1 (Klawatti Col to Summit)
From Klub Klawatti and Klawatti Kol, traverse 0.4 miles on the upper reaches of a lobe of the McAllister Glacier below the West Face of Klawatti Peak to the 7,900 ft Klawatti-Austera Col. In winter/spring it should be pretty easier to get over this col (there is a short rock wall; might be a snow bank at your pleasure). On the east side of this col is the Klawatti Glacier. Directly to your north is Austera Peak beyond a turret tower on its South Ridge. To the right, the very top of Primus Peak should be visible rising above the East Ridge of Austera. In summer/fall (particularly in late season), receding snow embankments and moat issues at Klawatti-Austera Col will make the crossing more difficult ranging from Class 5 downclimbing at the lowest notch to Class 4 to the right (SE) of the lowest notch. The latter can be reached by a snow bank then a step across to rock. The col crossing can be done unroped if you're careful about it.
From the K-A Col, the idea is to traverse north and northeast then east closely south of the East Ridge of Austera, descending along it to about 6,800 ft (see map). This is the first point where you can round the East Ridge in comfort. In early season, this is a 30-degree snowslope. I imagine later it is either heather or talus. The other side of the ridge leads to the North Klawatti Glacier (the fifth and final glacier you will be on for this trip). Negotiate crevasses (if any) to cross over to the Primus massif and simply take the easiest route to the summit (Class 2). The summit is a flat area with grassy patches amongst rock. Camp to summit = 4 hours.
Return the same route. Roundtrip = 8 hours. Total elevation gain roundtrip camp-to-camp = 4,000 ft.
Approach 2 & Climb 2 (Thunder Crk to Borealis Glacier)
This has been described by some as a brush bash. I can't say as I haven't been that way so I won't elaborate too much. Drive the North Cascades Highway (SR-20) to where it crosses Thunder Arm-Diablo Lake (1,200 ft). If coming from the west, this is about 9.5 miles from Newhalem. If coming from the east, this is about 1.3 miles past Diablo Vista. You really can't miss it. There is a campground there. The trail begins at the south end of the campground on the west side of Thunder Arm.
Hike the Thunder Creek Trail for approximately 6 miles (at 1.6 miles there is a junction with the Panther Creek Trail; do not take the Panther Creek Trail; stay right). McAllister Camp is located at 6.1 miles (c. 1,700 ft). At this point there is a confluence of two valleys (McAllister Creek on the west and Thunder Creek on the south). The lower North Ridge of Tricouni Peak is between these. Tricouni is Primus' neighbor to the immediate east. The idea is to ford or take a log across Thunder Creek and mount this ridge. The ridge is long, cliffy, and brushy but manageable. Continue up the ridge for 2+ miles to the toe of the Borealis Glacier at 6,000 ft. Good camping here. Allow an entire day to make this camp.
From camp, climb up the glacier on its eastern edge to the Primus-Tricouni Col ("Lucky Pass") at 7,200 ft. There is an icefall in the middle of the glacier that you will have to avoid but it is apparently not difficult to do so. The East Ridge finish from the col is also not difficult (Class 3 max). 3 hours from camp?
Primus Peak lies within North Cascades National Park, so park rules apply. Best to consult with the park service for specifics (they'll probably try and put a blue bag into your hands). A camping permit will need to be acquired though you could probably forego this for the north side approach (it is unlikely you'll see anyone). The south side approach can be crowded since it is the standard for climbs of Eldorado Peak. Party size limit is 12. Permits for this approach can be hard to come by (I guess). In early season when the Marblemount Ranger Station is closed, the permits are self-issue outside the building. Permits can also be obtained in Newhalem (I think).
When To Climb
This area makes for an excellent early eason ski tour (when coming from the Eldorado side). In early spring/winter, this is an outstanding fantasy land of glaciers and serrate ridges. You will be aroused by all the mountain poorn surrounding you. Otherwise, it is a summer or early fall kind of destination. Rock climbing for Primus Peak and its northern approaches is minimal. I won't speak for the northern approach along the North Ridge of Tricouni.
For the Eldorado Ice Cap approach, good camping can be found in Eldorado Basin (~6,000 ft), Roush Basin (6,300 ft), the Eldorado snow and ice dome (7,500 ft), the East Ridge of Eldorado (7,500 ft), or Klawatti Col (7,800 ft). There is not much point in camping farther out than Klawatti Col (unless, perhaps, if you will be going out a different way--like for a traverse down into Thunder Creek).
For the Thunder Creek approach, there is good camping at 6.1 miles along the trail (McAllister Camp; 1,700 ft). Beyond that, the next and only real suitable camp is at 6,000 ft at the toe of the Borealis Glacier (see map).
You can go here for a report on conditions in the area. Also, this page will be useful. The first (lower) part of the Thunder Creek approach does not lie within the park but instead in the Ross Lake Recreation Area. Even so, the park and the recreation area should be treated as one.
Marblemount Weather Forecast (nearest town).