I have tried all kinds of different creams and soaps and pre-shaves/oils. And I get decent shaves out of most everything I try, but after a few days, the irritation is back.
f your start is OK (with no irritation), then it is likely the blade that needs changing...try changing the blade after 2nd day or 3rd day (rather than usual 4th).
Also, Chris' advice on Derbys is dead on...they are hit or miss. Did you try any other blades before settling on Derbys?
I shower at night before bed. The irritation seems to start in the middle of the day, and is then worsened after I go over the irritation with my razor.
Hmmm...when do you shave then? The fact that you say that you go over the irritation with the razor leads me to believe that you shave after the middle of the day. If the irritation is already there before you shave, then it's probably something you use at night while showering or a facial cleanser that may be the culprit...it is irritating your skin. Try to use less product in shower and stick to natural shampoos/conditioners and bath soaps.
I think it is ingrown hairs that are the culprit. I make one pass and go with the grain. I have also used Anthony Logistics Ingrown Hair Treatment. It works at first, but when I start using it once a day (once every two days is recommended), the ingrowns start up again.
Maybe, maybe not...have you tried settling for DFS rather than BBS? or alternate shave days? Give your skin a break and time to recover before shaving?
I use a merkur 11c open comb with a derby blade. I previously used a 34c with the same result. I have a savile row brush and use mostly tobs avocado. But occasionally I will use nancy boy, and palmolive euro soap. I only get irritation on my neck, and only on one side.
Given the clues above, I don't think it's the razor
I change my blade every 4 days or so, but it seems to be better when I first put a new blade in.
See first comment above.
I think it's either:
(a) some product that you using (or overusing) that is irritating your skin
(b) blade...try changing to a different blade (if you alreadu have not tried sampler packs)