To stain unfinished wood picture frame mouldings, first, assemble the frame. Then sand the frame using a 220 grit very fine sandpaper to produce a smooth quality finish. Next, use a tack cloth to remove any fine sawdust from the frame. After this prep work, brush the stain onto the wood using a 1” foam brush. The foam brush is best because it doesn’t leave any brush marks. Brush on the stain evenly with the grain.
The stain we recommend is General Finishes Oil Based Gel Stain, available in 14 colors. Oil based stains are easy to apply. They provide a deep, rich saturation and can be made lighter or darker by wiping away the excess stain after application. The sooner you wipe away the excess, the lighter the shade will be.
When you are satisfied with shade, let the stain dry. Allow 24 hours. Then apply the stain again in the same manner and let it dry again. The more coats you apply the darker the color will be.
Alternatively, to paint your frame, we recommend Rust Oleum American Accents Ultra Color. Spray painting is faster in both application and drying time (as little as 20 minutes), but covers the wood in an opaque coat that conceals the natural grain. This can be beneficial when it comes to Poplar, which can vary in color, displaying patches of green, brown or purple along its length that may not match up when cut into four sections. More on using Poplar raw or unpainted.
Here’s another helpful article on staining. Wood Staining Guide for Beginners