That paranthetical bit should be factors of twenty and ten, not ten and five. Hit the Submit button when I should have hit the Preview button!
I have used a variac to bring back equipment that hasn't been turned on for a year or two, but decades is another story! My method is to start out at zero and bring up in either six or twelve volt increments (I use these values because they divide into 120 VAC by factors of ten and five respectively) depending on how long since the unit was last energized. Twelve volts if it has been a year, six volts if it has been more than a year. I also vary the length of time at each increment depending on how long since last energized, 10-15 minutes for a year or less and 15-30 minutes for more than a year. I would suggest you move up in 6 volt increments and leave it at each increment for at least 30 minutes. The whole point of this is to reform the capacitors, after decades they may very well be bad, but it's worth a try. One other comment, my variac has a tendancy to blow the circuit breaker when first plugged in if the load is attached, not sure why, but I start out with no load, make sure it is at zero volts, then plug in the load.
Lots of good, albeit varying, advice. My technique was based on the recommendation of a well known manufacturer who's name I have forgotten. I will say that technique is not one you would want to pay someone to do, but then if you are paying someone they will use what works for them. Given the amount of time these units were stored extra caution is certainly advised!