When my usual method of cotton-bud-with-De-Oxit didn't fix these crackly slide pots, it was time to open them up and take a closer look.
Note the Resonance pot is inverted with respect to all the others.
Unbending the 6 lugs.
The wiper tines are extremely delicate. I chose to very gently splay them a little so they would be in contact with a fresh area of track, being careful not to deform their angle from the carriage. The contact surfaces were also very gently sanded.
A gentle rub over the conductive track with some 600 gauge superfine sandpaper has taken off the darker layer of oxide. Don't sand the resistive track! The oblique lines across the resistive track are normal, presumably a result of the manufacturing process.
In my case, although the resistive tracks had been cleaned quite well with the De-Oxit/cotton bud, the oxide on the wiper and the conductive track meant the crackles persisted. They are now working like new.... but for how long?