That has always (regardless of the razor) been the trouble area for me as well. I use very short repeated (1/4" to 1/2") strokes in the direction of my jaw line, working my way down north to south on the first pass. The second pass I repeat the process south to north, and work my way back up my jaw line. So, while following the jaw line "on edge" (so to speak) might seem a bit scary, I haven't had an issue or any irritation (as might be expected). I take my time, being careful that the repeated strokes are light. Today's razor was the DE89L, and as with the others, that seems to work fine for me.
I have tried several ways to get this area smooth. One way I have found is to blade buff with an OC. I have a Parker 89R and 22R which wasn't getting the job done. When I use the Merkur 11C with Gillette Black Nija's after WTG, XTG, AGT, and Blade Buffing DFS to BBS. Try mapping your beard as well. This will help to identify just how to got ATG. For me is it more of NW to SE on the right side and SW to NE on the left. The school of thought is does SWMBO rub your neck or just your cheeks. Sometimes it best to get DFS all around with the exception to the neck.
I'm having the same problem. I'm using a merkur 38c with merkur blades preshave by washing face with nosema then applying a hot towel then Peraso preshave and end up using Peraso soap. I make three passes and have serious razor burn on my neck
I solved the rough patches on my jawline by doing two things: 1. getting an aggressive razor. I alternate between a Gillette NEW, which is on the aggressive side, and an open comb M.P. r89- also called a fatip. 2. using a technique I've heard described as "bullfrogging." To do this you drop your head down and cause the skin on your neck and jawline to puff out a bit. I can then get the jawline successfully. This doesn't work for everyone, unfortunately. As for the neck, I come to accept that I just can't shave some parts smooth without having some trouble. So I leave them slightly stubbly. I suppose I could map my beard or work on technique, but I don't have that much time to shave in the morning.
This works well and I use a aggressive razor. A Merkur Slant Bar, works every time. It is by far the best shaving DE I own.
For me with a de to get the jawline smooth is when I am doing a north to south pass I shave diagonally towards the chin and the same this when I am going south to north. Then if needed completely atg from ear to chin on the third pass. This generally leaves my jawline quite smooth. The valley of my neck is a whole different beast. No matter if I shave with a straight or de I need about 5 different angles to get it smooth and I really risk irritation if I go that far.
Just to give a bit of an update since I started this thread, I have tweaked and re-tweaked my technique and I think I'm finally honing in on what works for me. I do my normal 2-3 pass shave, and then a final pass focusing on touch-ups to teh jaw line/neck 'valley' area. I use very short, quick strokes, almost a blade buffing technique but maybe not quite, working my way up from my chin toward my ear. I've also moved on to blades that are a bit more aggressive than the Derby's I had been using primarily as well. I am finding that my skin is able to handle multiple passes without a problem, particularly in this area (lower on my neck can get irritated pretty quickly if I'm not careful on my technique) so I can even go back for a 2nd detailed touchup pass if needed without running into problems with most blades. I am not having another problem all together though- I am wanting to chase the baby! I've gotten DFS's from multiple blades and right now my go-to blades are Voskhods and Personna lab blues with the Personnas seeming a bit more likely to irritate my neck line, but also resulting in a slightly closer shave. But I still haven't been able to catch that BBS shave yet. I've tried Derbys, Feathers, Rapiras, Voskhods, Personnas, Treet, Gillette 7 O'clocks, etc... Is it likely that I might just need to move up to a more aggressive razor than my EJ 89L?
Hopefully not to late to the thread ... . I've found a Water Shave touch-up to work for me after WTG, XTG, and ATG passes to gain a BBS shave. Less irritation for me than making more than 4 full passes ... just a clean-up with the water shave. BTW, if you do move to a more agressive razor (like a slant) then you'd have to gauge whether a water shave clean-up would cause too much irritation or not. For me, three passes with no touch-up is about the max I can tolerate with my slant.
Correction: "... than making more than 4 full passes ..." should be "... than making 4 full passes...". (more than).