Thomas the surface rust you mention is actually rot also as mentioned it has been repaired previously to your ownership,preparation is key take Trikey advice and remove as much as you can so you can expose the full extent of the rot and the only way to prevent this coming through any repairs that you do is to remove it all back to solid metal then make good this will mean welding in a new piece then you can start your filling and repair process be careful not to sand with a heavy grade outside the area you are repairing as you are creating alot more work for yourself also not a good idea to use masking tape around the repair use feather edge masking tape to form a flip edge you can use the natural lines of the arch to make your blend good
On a positive its all good practice for you for when you start college
