hi Bill, the best way is to is unfortunately remove the radiator from your home have it sand blasted to a like new condition
OR
Wear a good quality mask and wire brush it.
Some folks even burn the old lead paint off VERY dangerious practice.
The chemicals I have used I found are completly useless.
The other option is go to a decent plumbing supply OR sheet metal shop and tell them the measurements of this radiator so a cover can be made to go over it.
Making sure you leave an opening for the cool air to enter the bottom and an opening on top for the hot air to circulate out AND A trap door for acess to your bleeder valve/air vent and shut off valve.
Have a great week end.