The correct acronym is POS! If it were me and I wanted to ensure a good seal, I would do the following.
1) clean as much debris form the mating surface as possible using penetrant and elbow grease. Any stubborn pieces of old gasket should be
carefully removed with a steel bar or utility knife.
2) tape a large piece of P220 sandpaper to a plate of 24*24" piece of glass (I use a spare from our greenhouse but it shouldn't be to hard to come by).
3) Secure the "sandpapered" glass to a sturdy, flat surface. Place the mating surface of the header flat on the sandpaper surface and move the header in an orbital motion.
4) Pause and verify that all of the surface is in contact (the sanding marks or lack-thereof will tell you).
Continue until the mating surface is smooth and uniform.
This will ensure a tight seal for the reinstall. It's not nearly as much work as it looks like in my instructions...
Good luck!!!