Rio Rondo's Paso Stirrups (items SP50 and SP51) are sold unfinished and require some added hand-work before they are suitable for use. This work, however is fairly simple and straight forward.
This is best be done using fine grit sandpaper (about 400 grit, wet or dry). Larger nibs or excess metal can be removed with a hobby knife or carbide scraper.
These areas include:
Be very careful as you work. It is very easy to mar or scratch your paint while you work so don't get in a hurry. The paint is more likely to be marred if it's not yet dry as you apply the next layer of paint, so its best to be patient and allow each layer to dry before proceeding.
When you have the dark brown "wood" color of the stirrup even and are satisfied with it, allow the paint to dry for a couple of hours. (Longer if the weather is humid).
When you are CERTAIN the paint is completely dry, I recommend applying two coats of a clear matte nail polish, or other matte protective spay over the simulated wood-colored areas. This gives the "wood" colored areas a satiny, burnished look, and protects the paint from future marring. Allow the first coat to dry about an hour before applying the second coat. Allow the entire stirrup to dry at least another hour or two before attaching it to the stirrup leathers.
Alternatively, you can use a Matte Spray Finish to help protect the surface.
Paint will stick to pewter if you are very careful not to rush the process and allow the paint to dry thoroughly. Applying the matte polish over the top will help seal it (but is only truly effective if the paint is totally dry) and making certain the polish is completely dry will ensure a long-lasting and beautiful stirrup.
Should you ever need to re-do your stirrup at a future time, simply soak the stirrup in nail-polish remover or acetone (please read all precautions on these materials) for an hour or two, then wearing latex gloves, use cotton swabs or kleenex to remove the paint etc., and work it out of any crevices. Wash the stirrup with soap and water, and allow it to air-dry thoroughly before repainting.