Classic 3 VPI

My Classic 3"s belt squeals upon turn on.  After a couple of seconds, the squealing stops.  I've tightened the pulley, installed a new belt and did what VPI recommended, I powdered the belt.  After applying powder, the squeal quietened but squealed again on the very next try.  I reapplied powder and the same thing happened.  Any advice is welcomed.
My VPI prime does the same thing, see the thread I put on VPI's general audio forum.
My VPI Classic 3 belt does the same thing. I powdered it as recommended by VPI but as you said, it doesn't last long. I finally got used to the sound which stops after the platter gets up to speed and use it that way since it doesn't affect the sound on the record. I still love the TT.
You have probably transferred some "natural oils" during handling and now they have coated all surfaces.

Powdering the belt is a temporary fix and does not address the real issue - belt slippage - you need more grip - the powder allows the belt to slip silently until the platter is spinning - result, no squeals, but the powder wears off and makes a mess on the plinth

You might want to consider cleaning the belt and all surfaces that come into contact with the belt thoroughly (I've used isopropyl alchohol) it will give the belt more grip and hopefully eliminate the squeel.

I've had this issue for years on my "Rega" and cleaning has yielded the best results, but  if that does not work, you will have to resort to the "Rega nudge" on startup - you just give it a little push and then turn the power on.

If you change speed a lot I would recommend getting a pair of cotton cloves when handling the belt.

I do agree with other posters about running multiple belts - that is a big platter to get moving and one skinny little belt probably will not provide enough grip.

Regards - Steve

My Prime gives a small squeak upon start up with no assistance.
I've gotten into the habit of giving the platter a little help when pushing the start button, and there is no noise now.
Just a slight clockwise push on the platter while engaging the motor.

I know of other turntables that require this to get their heavy platters moving.