Adie's shot blasting is definitely the least effort solution, you could also get it painted by whoever shot blasted for you. Spraying is always my issue as I have to do it outdoor and my garden is like a wind tunnel.
Al makes a good point about paddle discs, it's easy to grind into the metal and make a mess. Having said that I stripped the rust off a drum with one and got the finish spot on, a couple of slight burns and scars but really nothing major.
I personally don't find cup brushes any good, I've tried them on my angle grinder and on my drill but found them ineffective. Could be user error though
So if you do go by hand and don't fancy risking the paddle disc I'd go with Al's other recommendation and try a wire wheel.
Just thinking as well, I've seen another type of rust removal disc,
http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_categoryId_165557_productId_161123_langId_-1?cm_mmc=-_-Workshop+%26+Tools%2FPower+Tools-_-Halfords+100mm+Paint+%26+Rust+Remover+Disc&source=shopping&istCompanyId=b8708c57-7a02-4cf6-b2c0-dc36b54a327e&istItemId=lwmmix&istBid=tNever used one of these myself, but looking at it I would imagine it's very unlikely to damage the good metal. Al you ever tried one of these?