c4 grand picasso sill rot

[614cooker]

Hello,
I bought a 2008 c4 Grand Picasso with full service history which i paid £1,100 for in 2019 which I thought was a good but at the time. 2 weeks after buying the car it started cutting out everytime it passed 3000 rpm which I have fixed replacing the fuel pressure sensor and had a few rust bubbles on the both sides of the sills, I sanded the sills down and removed all rust spots which I sanded to fresh metal then primed it with antirust primer and painted with matching paint which came out like new, within 6 months the issue has since returned but only on the passenger side, I have scrapped some rust off and the sills still look fine under the rust. I am going to sand it down again but need some advice on how to stop the issue happening again. I honestly thought sanding it to fresh metal and treating it with antirust primer would solve the issue but only made things worse.

So my first question is I just need some advice on what I could do to stop the rot apart from antirust primer is there anything else I should be using to treat the metal.

Also while im on the subject I would like to ask if anyone has any idea on the costs of cutting out the rot and welding a new panel in the issue is at the front of the passenger side sill which measure around 25cm in length. I understand that costs vary but a rough cost is all I ask.

Regards, Matt.