I doubt that would work, wq. the preparation for POR 15 is pretty prohibitive, and the cost of enough POR 15 to coat the inside is prohibitive. It's great for paint on applications, but not for shake and bake. If you don't get it on EVERYWHERE it will rust in the areas that are not protected, and slowly make it's way aroung the POR. If you are gonna do something I would suggest a small silica gel pack (like they have in camera packaging) taped to the inside of the slider before welding. that'll do a good job of absorbing any extra water vapor.

It's really not worth the trouble, as FSR said, it'll take eons before the inside of a slider rusts out. it'd have to have ounces of water, not droplets.

Matt