The ones without the capacitor don't lose OCV in my experience. And yes, most of the ones with bad caps will still run one lamp.

How complicated it is to disconnect the capacitor in the ballast and tap the "before capacitor" wire to outside ? Can it be done with some minor digging into the tar with a knife, without the big work of melting and repotting it ?

Then you can connect a new capacitor from outside, between the new wire and one of the capcitor side wires (Reds ?)

It's probably harder to do on some ballasts than others. Not worth the effort unless you really feel like doing it for whatever reason.

Then what about opening the back of the ballast and driving a self tapping screw through the capacitor, to make it into a short circuit ?

http://www.lighting-gallery.net/gallery/displayimage.php?pos=-121294

If you do that it will just keep blowing the line fuse Everytime you switch on

The capacitor in US Rapid Start ballasts is not across the input AC as in UK. It is in series with 2 lamps, if we disregard the heating circuit. If we short the capacitor inside the ballast, we can connect a replacement capacitor outside. Only problem remaining the is how to supply the heating

I'm reading all of these replies, and I just have to ask.  Wouldn't it be easier to just replace the ballasts? 

Yes.

Well, try cleaning the lamps and channel cover first as it is easier than wiring a new ballast. If that fails (you'll know in a couple of days) then a replacement is in order.

As for fixing a potted ballast, it is possible as there are some members here that have done it however it is quite an undertaking and not something I would do. You can get a NOS rapid start magnetic ballast on eBay or a new electronic ballast for T12 in hardware stores.

I've had similar problems with  f96t12HO and a f48t12HO rapid start but it wouldn't come on between 40-50 degrees they had tons of hours and I used on a daily basis it was very weird the ends would glow orange then I'd tap it the f48 would come on but not the f96 I just replaced the ballast and all was good. Your situation sounds like a starter or moisture you should experiment with it and see what it could be?

It tends to happen when the humidity is high outside, which equates to high humidity inside.  Cool weather, they work fine.