Step One:
First of all, find documentation to support the finishing detail of your model. Replicating a full size airplane is what this is all about. Take the time to find clear photos of an airplane to duplicate in miniature. Photographic documentation is your best source of accuracy. Artists take great liberty with three view drawings. Here's a good example of photo documentation supporting a stainless exhaust panel.   
Note: Stainless plate behind and below exhausts.
Photo Courtesy of Curtis Fowles @
Step Two:
What we're after is duplication of your documentation, not what you think it should look like:^) If you do not have photo documentation it is virtually impossible to replicate the look of stainless steel. Mask off perimeter of panel with Scotch Fineline® masking. Now you scuff the panel interior with a Scotch Bright® pad. Scuff surface in a single direction. When surface appearance is scratched so it does not look at all like surrounding panel, remove all masking.
Step Three:
You are now ready to color the stainless steel panel so it will match the photograph. So you'll know that all P-51's don't look alike, here is another stainless steel exhaust panel photograph:

Photo Courtesy of Curtis Fowles @

Remove aluminum residue with Prep-sol. Re-mask the perimeter of the stainless exhaust panel with Scotch Fineline® masking. You are  now ready to apply a color to match your documentation.

Color match your panel with dirty clear lacquer paint or stains. Dirty clear is by far the easiest to match because it gradually darkens as more and more is applied in light coats. Select color pigment which matches your photo documentation in its opaque state then thin with 80% clear. If this % is not sufficiently opaque, reduce clear %. I would thin the paint such that it requires no fewer than 4 coats to darken.

Step Four:
In rare cases, stainless steel exhaust panels can be replicated with a single coat of matte or flat clear paint applied to the masked area. In the case of artist drawn 3 view documentation...this usually works.