Take a good look at the alternator and crank pulley to see if the grooves are worn.

Belt dressing is supposed to soften the rubber usually with sort of a solvent action. There is a rubber cement type gasket cement called Gasga-cinch. NAPA sells there version of it. Besides the use for gaskets it also has belt dressing.

As the other said belt dressing is entirely temp. Better to get to the source of the problem.

Note that a former owner may have used US sized/widths of belts. Which of course can wear the pulleys in the wrong places.

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84 300D, 82 Volvo 244Gl Diesel