In trying to change the brake pads on my mountain bike, I managed to get oil on my new pads and empty out all the brake fluid, and consequently also get some oil on the disc. I took the bike to the shop and they are now telling me I need to change the pads and the disc. I can understand the pads, but do I really need to change the disc or is the guy (girl actually) just scamming me. The disc is not soaking in oil, from what I can tell it just has a bit of oil here and there.
First of all you need to tell the LBS (local bike shop) that the disk can be "fixed". Then you need to find another LBS because they are either amateurs or are simply trying to make you buy stuff that you don't need.
You need to remove the oily disk from the bike and use a bike degreaser or alcohol on it to remove all oil. Rub with a clean dry cloth or kitchen paper and repeat. While finishing the process avoid touching the disk with bare hands on the face where the pads contact it because fingers leave grease as well.
Regarding your pads, these can be "fixed" as well in case they are sintered. You'll clean them using a degreaser or alcohol and slightly grind them (let's say remove ~0.2 mm of surface) with a thin sand paper. This will make sure that the pads are like new. Then you'll do whatever you do when using new pads (bed them in).
If the pads are not sintered but are organic then the above process may not do the work because organic pads can suck some oil inside (like sponges but not that much). I've heard of people boiling the pads in a frying pan with tap water and a bit of dishwasher soap but I don't know whether this will work. If you do that make sure you don't inhale any of the steam and also use an old pan which you'll never ever going to use again for cooking.
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Just to add to the top answer - brake cleaner (sold by bike shops and car service places) is the ideal product for cleaning your brakes - just spray it on liberally and let it drip off.