On Thu, 12 Jan 2006 23:41:23 +0000 (UTC), Christopher Tidy
wrote:

You _might_ need to de-rust the laminations, just to get the stack
height small enough to fit back in the coil former. If they're really
rusty, then they're also likely to hum more than usual.

You should re-varnish too, if they're particularly scabby. Missing
varnish increases losses in the core, leading to wasted heat.

I've thought about electrolytic de-rusting, but haven't tried
it before. Does it work okay with a whole bunch of little components
resting in a metal basket?

Yes - a rotating basket would be ideal, but just turn them over every
few hours. You need a big shallow basket, and to have the anode on the
lamination side of the basket. Generally a plastic basket works best, if
you can arrange good contact to the workpiece. A conducting basket tends
to steal too much current flow for itself.

Electrolysis may also leave the varnish intact, compared to abrasives.
Or it might strip it - you don't know for sure until you've tried that
particular varnish.