Found a decent anvil for a good price (and a long drive), cleaned the rust off of it, and am considering trying to flatten the top a bit, as it has about 1/8-1/4" dip in the middle...I've read several threads saying to use as is, some sway is actually useful, etc...am I worried about nothing?  My other thought is that if I absolutely need a fla surface, I can make a hardened plate attached to a hardy...appreciate any thoughts.

I wouldn't worry about a bit of sway especially that small of an amount. Just use the anvil and don't sweat the small stuff!

Without telling us WHAT YOU PLAN TO DO WITH IT we can't say.  However in general smooth is much more important than flat.  

Also most methods of flattening an anvil that don't cost more than buying another one in better shape are really ways of destroying your anvil.  Some anvils used a high grade of wrought iron which is quite soft and so tend to get sway backed more than others---but the thickness of the face is still the same.  Any milling or grinding to flatten such is about as smart as taking a sledge hammer to a Maserati because one of the tires is low on air...  It can be built up by welding *WITH* proper preheat, postheat and correct alloy rods---see the Robb Gunter method of anvil repair---but this often is more expensive than getting another one.   Generally we advise folks to use their anvil for a year before thinking of making any changes.  Often folks decide that it's flaws are really features and are upset when they use one without them...(My flattest anvil has a swayback one right beside it so I can use it when straightening blades.)

What I have usually seen is that they are only swayed from end to end, not side to side. Most forging is done side to side not end to end. It is flat in the direction most forging takes place.

I wouldn't touch it.  If you do, you're likely to grind through the remaining face plate.

Nothing wrong with a bit of sway, especially if you don't have much experience forging.

What does the sway keep you from doing now?

What Biggun said mate, you work across the anvil not along it, and if you need to straighten a bend then you can use the hollow to your advantage, use it as is.

I once went fishing with a work mate, he had all the best gear and ridiculed my cheap second hand gear as I was just starting out , at the end of the day i caught much more fish than him.if you are in no hurry listen to the story your gear can tell you before you rush to buy new.

There is always a flat section fore or aft of the sway section. However the sway section is usually flat from side to side . I like the sway in my PW.

Personally I hate sway on an anvil and wouldn't touch it with a barge pole. BUT as others have said trying to fix it will most likely ruin the anvil. Use it as it is and just crack on making things.

 

All the best

Andy