Then sand it down with 80g as far as you need to get it as smooth as you need.
As long as the paint that's there isn't coming off and your not hiding rust (even on the edges) then just work off that.
Only reason for a sealer now is to keep your volatiles in your fresh paint from reacting with your old paint.
A quick test of your new paint in an inconspicuous spot will tell you if you even need to worry about it.
If your old paint lifts easily you'll have a clue as to it's long term properties I guess and the make decisions from there.
