Ah, had those on the Mazda Minivan...big hassle. IME it simply *does* take some time/effort to do `em properly, and some such grilles are made from stuff that`ll mar pretty easily.
I have various sticks that I wrap MFs around, some even have MF sleeves that fit over them. Yeah, hassle doing every individual slot, but it`s just what needs to be done.
For the first pass(es) a long-bristled BHB can be great for starters, especially with a foamgun/etc. providing lots of soapy suds.
If you take the time to do the grille with, say...KAIO followed by multiple coats of KSG or (fewer) coats of FK1000P, and use some leaves-stuff-behind product for your Drying Aid, the slots will not only look great, but will also a) not retain as much dirt and b) clean up easier.
IME dressings don`t last, sometimes harbor dirt, and some even run in the rain, so I gave up on those years ago...but I hear there are better ones out these days (Tuff Shine comes to mind). Still, IMO you gotta get into every slot, paying attention to every surface/seam. Good lighting helps.
