Remove any excess rust if you can, as suggested with a wire brush and then spray lightly with CRC.  This will pretty much stop the rsut coming back for a fair while, you don't run the risk of any new paint peeling and especially if the transformer does heat up a little, the CRC will soak into the metal and do a great job.  Quick, cheap and easy.  I've done this on plenty of transformers now and I haven't had any problems since.  It leaves them looking nice and clean and original.

If you don't want to spray CRC around and get it on the cabinet, just wet a rag with the CRC and wipe it on.