I have a pair of new aftermarket front fenders for my 93 F150. They have the typical black primer, but it seems to be something permanent as I can rub lacquer thinner on it and it doesn't come off. I know to use epoxy primer on the outside of the fender after scuffing it up, but what do you do about the inside of the fender...to many nooks and cranny's and covered areas to scuff it up...is it necessary for the inside to prevent rust? I hate using undercoating on stuff as it cracks in time and doesn't do anything in the end.

 

For the inside of fenders you can spray the same epoxy. Just make sure it's a sealer.

Or you can use a brush on paint like a rustoleum. But you still have to clean and prep the area to make sure it sticks.

You can also coat the inside with fluid film or something like that.

Make sure you the inner fender clean of tree leaves, etc.

 

SEM makes a sprayable,black undercoating in a large aerosol can that works great for treating inner panels like that.It will stick to about anything and is paintable after being primed or sealed.I have never had it crack or fail in any way and has a nice even texture when sprayed.I have used it literally hundreds of times on customers vehicles. I would spray my inner fenders after lightly scuffing what you can get to. Good luck!

 

The black coating is an eletro-plating material, something similar to what the factory uses for its original fenders.

I'd use the very same exterior finish on the inside, making sure you break the coating where it can be overlapped by the eventual exterior color finish. Since this would adhere so well to the exterior no reason it shouldn't do the same on the inner surfaces too.

Maybe something like bed liner instead? There's a nice DIY product called Monstaliner seems promising: Monstaliner do-it-yourself roll-on truck bed liner outperforms Herculiner Bed Liner, Duplicolor and Rustoleum Road Warrior

As always proper preparation is vital to the long term effectiveness of any exterior finish.