How best to fix this?

[handpaper]

Pic is of the inside of the left rear arch, looking forward. I'm not sure whether to fabricate a piece, buy an 'outer inner arch' (?) panel, or clean it, seal it with rustproofer and fibreglass it
I'm looking to keep the car for a few years and apart from a small part of the rear valance, this is the only bit that needs repair.

[oylegate]

I would buy an outer panel and fabricate an inner one. Wouldn't go near Fiberglass if you plan to keep it for a while.

[stevotheevo]

just done this myself on all 4 corners, fabricated all of it.

not a quick or easy job, and doesnt look very pretty but its a bit of a slag alround so wasnt bothered.

if you want it looking minty buy the panels.

[sam325is]

Chop it back to clean metal and blend it in with fiber glass.Id like to know why people think its such a bad idea. IO would have fiber glassed mine instead of welding them if i had the idea at the time. My holes werent quite as big as that though

[oylegate]

The problem with fibreglassing an area that big it that the fibreglass tends to collapse into the hole unless you use some sort of reinforcement. AKA CHICKEN WIRE!

[abelai]

I replaced the outer wheelwell £70 ish from bmw you will have to cut the lip off the rear quarter/wing to fit it but that looks rusty anyway, the repair panel for the arch lip is about £30 for a late model and £15 for an early model hadrian panels are the only ones that sell the late style don't believe anybody who tells you different.

[sam325is]

fiber glass it from the inside first. or use some sort of platic to mould it on. not chicken wire. dont want more rust now do we.