Used inner fender wells

[7D2N]

I found a used original I'll make work, (cost savings) figure I'll paint the top side but how should I deal with the under coating? do I need to sand down or re coat ??

 

[Big Blzn]

I'd try a stiff wire wheel on a grinder. It should get all the rust off and do a pretty damn good job on the undercoating too. Then just spray or roll with an good rust inhibiting paint like POR, Zero Rust, etc.

 

[fishinnutt]

I have been using BIX Spray On Stripper and it will make quick work of most things you might find on a vehicle.