Rust on wheel arch of mk4 golf

I was out cleaning the car today, (day off work woohoo) and to my shock
and horror on the wheel arch mosto f the way around it on a mk4 golf is
rusty spots, paint is cracked. I accidently scarped it on a wall a few
years ago. I got it fixed by chips away and looked good as new and
couldn't see any damage. .. Till now.

I assume he didn;t do a proper job or something. Tried contacted them
but the guy won't come out and look at it as he says no job he has ever
done has gone rusty and said good day sir and slammed the phone down.

Called head office to deal with him as it was feckin out of order.

Going with the thought that they will no fix it. How can I sort it out
myself. I can spray have a gun, compressor, etc etc. Just never dealt
with rust.

SO any advice on what to do first or the order of how to fix this kinda
of problem out before my car crumbles into pieces :)

thanks