Rad Sludge

My rad has been flushed and antifreeze changed a couple times over the years. It seems like there is always some tan colored coating around the
neck and cap.....any ideas where its coming from. I have a '96 3.8 Winny.

ps. its been doing this for years.

A lot of it is just minerals coming out of solution from the tap water shops use in the 50/50 mix, normal corrosion from the cast iron engine block & aluminum components, or sealing materials. Just make sure it doesn't get oily or you dont start loosing coolant- signs of a head gasket failure. Swapping out the coolant is a smart thing to do regularly. Many people totally neglect this.
G/luck
Joel