My experience with stiff (hard to turn) controls such as volume or tone is that the problem is NOT in the resistor section so spraying anything in there doesn’t help and may actually hurt by causing a different problem. The issue is usually that the lubricant between the control shaft and the mounting bushing has hardened. To cure this, tip the radio with the control shaft turned “up”. Warm up the shaft/bushing with a heat gun or soldering gun (don’t get all crazy and melt anything). Once it’s warm to the touch, put ONE drop of penetrating oil between the shaft and bushing and gently work the shaft by turning it until it frees up, then follow that up with ONE drop of a light oil and work it in. This usually fixes them permanently.
NOTE : if you use too much oil, it will flow thru’ and contaminate the resistance strip.