Paintbrushes are too flexible to really worry at caked dirt, but cut them down by a third or more and they'll develop enough back bone to be of some use. Wrap a few layers of duct or masking tape around the metal binder to prevent scratching anything and it'll be good to go. I buy the el-cheapo ones at the big box store and toss them after a few uses. In standard lengths paintbrushes work really well on things like engine cases, radiators, oil coolers and wheel rims and are handy for reaching in between the cylinder fins. You can also use a clean one, or a stiff hobby or artists brush to remove waxy residue from the spots you can't reach with your polishing rag, for example from around the edges of a tank badge.