88' MR2 - Rust in the rear wheel arch...help?

My car has a little rust in the rear wheel arch..its in an ackward spot...Its actually between the two pieces of metal that form the lip of the arch... Its propogated about half way in. Does anyone have any tips to halt this rust? I could use a pick and try to clean it out and slap some por15 in it but theres a chance it wont reach all the way in and will continue to rust. Or I can cut out that half of the lip and then use por15...Anyone?!?!
I appreciate any other suggestions!
Thanks,Matt