Technical how to solve this.

[Leroypanda]

 

[jrkitching]

Take off the wheel arch liner, remove all rust and flaking paint (a spot sandblaster could be useful here), prime & paint - then underseal the area behind the liner before refitting. At least it's solid white; possibly the easiest colour for DIY refininshing. For small touch-ins like this, I've found a modeller's airbrush gives better control than a rattle can.

It would not surprise me if removing the liner reveals a buildup of accumulated wet debris.

You might also want to inspect the other side, and other vulnerable areas, for signs of a more widespread problem. If this is the only rust patch on the car, I'm wondering if the wing has been replaced at some point?

Out of interest, how old is the car?

Interestingly it's the second picture I've seen this week of a Panda showing corrosion on the galvanised bodyshell.

 

[Leroypanda]

2013 car, theres also surface marks on the seam under the front wheel arch behind the shock and where the engine mount is next to the camcase.
Im really not looking forward to removing the arch liners they are a right pita.

 

[jrkitching]

Im really not looking forward to removing the arch liners they are a right pita.

Unfortunately there's no other way of doing the job properly.

 

[Leroypanda]

late spring job i think.

 

[Herts Hillhopper]

I know this won't help you, but will help others...
Regularly during the winter, and especially at the end after the last road salt's been washed off the roads by rain, have a really good blast around the wheel arches with a hose nozzle, including the lip nearest you above the wheel (don't need a pressure washer for this). This will dislodge salt-laden mud and prevent it from holding water against the metal during after it rains. That particular area of the rear arch lining is open at the base, so you can point the hose jet up in there too from underneath . Do the same around the spring cups and rear suspension beam, and around under the front arches, especially the springs . Also the little ledge between the front wheel arch and under bonnet where a frightening amount off mud collects. Finally, open the bonnet and aim the jet into the area beyond the bonnet hinges and down toward the part of the wing that meets the front doors. Watch and you'll see mud, leaves and other crud flow out from the join between the wing and the sill. All that may when damp is the perfect breeding group for the Tin Worm.

 

[FiatMatt94]

I know this won't help you, but will help others...
Regularly during the winter, and especially at the end after the last road salt's been washed off the roads by rain, have a really good blast around the wheel arches with a hose nozzle, including the lip nearest you above the wheel (don't need a pressure washer for this). This will dislodge salt-laden mud and prevent it from holding water against the metal during after it rains. That particular area of the rear arch lining is open at the base, so you can point the hose jet up in there too from underneath . Do the same around the spring cups and rear suspension beam, and around under the front arches, especially the springs . Also the little ledge between the front wheel arch and under bonnet where a frightening amount off mud collects. Finally, open the bonnet and aim the jet into the area beyond the bonnet hinges and down toward the part of the wing that meets the front doors. Watch and you'll see mud, leaves and other crud flow out from the join between the wing and the sill. All that may when damp is the perfect breeding group for the Tin Worm.

I know this won't help you, but will help others...
Regularly during the winter, and especially at the end after the last road salt's been washed off the roads by rain, have a really good blast around the wheel arches with a hose nozzle, including the lip nearest you above the wheel (don't need a pressure washer for this). This will dislodge salt-laden mud and prevent it from holding water against the metal during after it rains. That particular area of the rear arch lining is open at the base, so you can point the hose jet up in there too from underneath . Do the same around the spring cups and rear suspension beam, and around under the front arches, especially the springs . Also the little ledge between the front wheel arch and under bonnet where a frightening amount off mud collects. Finally, open the bonnet and aim the jet into the area beyond the bonnet hinges and down toward the part of the wing that meets the front doors. Watch and you'll see mud, leaves and other crud flow out from the join between the wing and the sill. All that may when damp is the perfect breeding group for the Tin Worm.

All I have is just a really good 9 litre garden pressure sprayer which I fill with snow foam & away I go, enough to go around the car 2-3 times & a couple of times up in the front & rear arches including lips & also in that little ledge which I have to say since I put Dynax S50 in there 2 years ago is still holding up really well as hardly any mud gets in there & every time I clean the car I always make sure it's not clogged with anything & water flowing out freely. The rear beam seems to holding up good now since I covered it in Vactan in the summer, still looks like new even after winter & a good pressure wash underneath to get the winter crud off, not that there ever is a lot as the entire underneath & arches have been Bilt Hamber'ed, didn't go to mental with the Bilt Hamber around the rear arches just at the bottom where the crap collects which is now a thing of the past since applying the wax, it sheets water off like mad