Basically there are three types of surfaces to clean in vehicle wheel wells:
1) Plastic liners
2) Factory undercoating on sheet and frame members
3) Fabric liners
I despise fabric liners in wheel wells. They hold road grime tenaciously and they are difficult to clean BY HAND. So those of you with electric or small-engine powered power washers have a REAL advantage getting this particular wheel liner material clean.
The other problem for me is getting my hand in there to clean the liner or undercoated member clean with my old cotton chenille pad between the member and tire with the tires remaining on the vehicle, as I assume most Autopians do during two-bucket car washers. For sports car and some sports/performance sedans, as the tire-to-wheel well clearance is minimal, cleaning wheel wells is impossible with the tires on. I find brushes do not adequately clean road debris that gathers in those areas, hence the chenille pad with my hand. I prefer to use Meg`s Detailer Line D101 All-Purpose Cleaner as the cleaner, along with the soap from the car wash (Megs Detailer D110 HyperWash Shampoo). If it is really dirty, I may use Optimum`s Power Clean.
I do check for road tar and road line/striping paint before doing so, as tar will get on my chenille pads and tar is hard to get out. For such tar I use Stoners Tarminator or 3M`s Adhesive Remover on an old cloth. For road paint I use Goof-Off paint remover (not to be confused with Goo-Gone), but it must be done carefully, as it will remove the factory undercoating or the coloring on fabric liners if wiped aggressively or saturated.
And cement/concrete specks? I use regular, not Diet, Coke-Cola soaked on a rag. Yes, I know there is a specific product for this, Back-Set by RoMix Chemical formerly sold by the now defunct Top of the Line Detailing Supply. The regular Coke does work if it is a small amount and has not been there forever. Some soak the cement with WD-40, but I have not done this, so I cannot vouch if this really works as well.