Cheese – Waxed cheese oozing oil

Could be an unfinished roux (the butter, flour mixture). But most likely it's because the cheese was heated too quickly or too much, causing the protein to clump up.

Suggestions:

• Melt with less heat
• Use a double boiler (to reduce hot spots within the pan)
• Toss the shredded cheddar with cornstarch first (starch helps reduce clumping)
• Add cheese in smaller batches (easier to maintain correct heat level and stir cheese in)

Cheese sauces will curdle more easily if they are not acid enough.

I struggled with homemade mac-n-cheese until someone pointed this out to me. The wine that Zippy suggests is one solution to adding acid without undesirable taste.