I got an as-is bike used that came with disc brakes. Both brakes squealed so I cleaned both rotors and both sets of pads. That fixed the back one but not the front one. I removed the pads and noticed that there is something that looks like grease in the holes of my front rotor. I used a q-tip and rubbing alcohol to clean both the pads and rotor and that quiets it down for a short time, until the grease re distributes over the rotor surface.
I'm sure someone will be able to provide a better answer before too long, but my solution for this sort of situations is baby wipes. I've used them on various parts of bikes (and babies) and they work really well - I think it's mainly thanks to their ability to hold/absorb substances. Things to bear in mind:
- We use pampers sensitive and I assume most brands will give you similar results, but I do note that pampers sensitives are fairly flossy in texture whereas some brands like huggies are more cloth like. Hope it's ok to use brand names on stack exchange?
- I imagine it may take some experimentation before you find a way of dealing with the shape of the rotor, feeding wipes through holes, getting into awkward corners etc.
- You may also find they leave some floss on, it might be worth clearing that so that you don't jump from the frypan into the fire - the wipes may have not been tested under the pressure and temperature present in disc brake systems. If the floss is synthetic it could melt under heat and become harder to remove.
The grease-like substance on the rotor holes could be from the brake pad when you squeeze on the lever. I suggest buying a fresh set of pads.
As for the rotors, dumb them in a tub of degreaser (bike-specific or industrial). After a while, get a toothbrush and scrub away at every surface and corner of the rotor while submerged in degreaser (wear gloves please), then leave it for a bit more.
Rinse the rotor with water then wipe with isoproryl alcohol and leave to dry. Dispose of degreaser properly or get a coffee filter and recycle it.
Attach rotors back to the hubs (don't touch the braking surface), install new pads, and see how it feels.