Cricket,
Here is a trick I tried with Rusty, my rusted 99K. His thread path was rough with rust and that was causing trouble.
So I got some really thick thread, probably carpet thread, coated it with jewelers rouge and then ran it through each guide as if it were the machine doing it. I used a back and forth stroke with the string until it broke from the friction. Then I cleaned out the thread bits and rouge and noted a huge improvement. It's just and idea to try for the burrs if you still have some.
Joe