I have to say ds2's rashes started out in little bits and pieces that I ignored for the longest time as well. I was kinda waiting for him to outgrow it, but at some point everything just became exponentially worse and strangers were staring at us. It was really hard to find a cause when it kinda crept up like that, unlike ds1's dairy intolerance which was really clear to see from the start and hence to get a handle on.
The mums here gave me some good ideas on what allergens I was not yet eliminating and we've added corn/gluten/sesame/sunflower to our forbidden list. If you google Dermatitis Herpetiformis, you'll have a good idea what my kid's rashes look like. It is finally clearing up now, thank goodness.
He has also been tested positive for dustmite allergy and I've been wielding a vacuum cleaner like a weapon. It must be HEPA certified, fully sealed unit to be effective against dustmites.
Re medicated steriod cream - it did provide quick relief and a good night sleep for a while, and if you don't have to use it long-term, it can really help. But in our case, because the underlying cause was not removed, each subsequent flare-up was much worse due to thinning of skin and lowered skin immunity. His skin became really fragile, tore easily, wouldn't heal for weeks and was easily infected. It was a vicious cycle especially since even cleaning the broken areas was very traumatic for everyone.
Re secondary infections - if it's a prolonged rash, check for secondary infection which may be bacterial or fungal. Ds2 had patches of secondary fungal infection which made everything very bad at one point last year. The treatment for fungal infection is the opposite of eczema advice so that's one good thing to check on (keep skin dry, no creams, no bandage that can trap moisture and cause more breeding). One friend whose son had very bad eczemic looking skin despite going to many many doctors finally found relief when a dermatologist started targetting fungal infections instead of eczema and also told them to stay off all seafood, including fish and seaweed.
Our allergist separately also told us not to use antibiotic cream (which another dermatologist had given us previously) on ds2's rashes because it will develop resistance due to the long-term nature. Instead we now use antiseptic solution to clean the skin everyday.
Hope you find something useful in this.