Back on my soapbox, so pardon the broken-record effect. For cleaning the lower areas and the undercarriage, I prefer Griot's long handled boar's hair wheel brush (pn 92547).
The problem I've encountered with flagged plastic bristles is that the flagging wears away leaving scratch-inducing UNflagged bristles. At least inspect them very carefully before/during each use. I'd rather just use the boar's hair and not worry about it.
Heh heh, am I the *ONLY* person who uses clay to remove tar? I can't imagine using a tar remover and then having to redo my wax or sealant every time. Just cleaned some nasty, hardened tar off Accumulatorette's A8 with clay. Worked like a charm. I DID wonder if that one would require a solvent (and subsequent rewaxing), but it didn't. I *CAN* imagine it being so bad that clay wouldn't work, but only if you left it on there for quite a while. Anyhow, just my experience...