The wire wheel may have taken off the high spots, but it will not produce a flat surface. Keeping it flat is just as important as getting the high spots off. I can't see the whole thing in your picture, but I'd get a flat tool of some sort, maybe a block of wood or metal and wrap some sandpaper around it. You'll be able to see what is going on from the marks it makes. ..... Now, if you've already made that surface a little concave with that wire wheel, there's not a lot you can do about it. Just get as good as you can. It should be okay because you have some unaffected surface that should seal.