How to get a smooth finish on a large picture frame with white paint?

IF it is rough to begin with ,sand it first. IF not, spray paint with light coats. Let dry, spray again lightly. Do again if necessary. Light sweeping strokes will keep it from dripping.

Use a foam brush or long sweeping strokes with a paint brush.

I’m going to answer this as though you are using white paint to redo the frame. It will be ultimately easier to remove the mirror from the frame, but if you don’t feel comfortable doing that, just mask the frame with painters tape. The way you wrote this sounds like you have a less than acceptable finish presently, so the first step would be to sand the current finish until you have something smooth to work with. Make sure you finish your sanding with something about 150-grit. Spray (or brush) a good quality primer, such as Kilz. Now begins the important work to wind up with a smooth finish. You have to do what’s known as ‘finishing the finish’. Take a piece of fine sandpaper, at least 220-grit, and lightly sand the primer smooth. NOTE: With sandpaper the higher the number for the grit, the smoother the sandpaper is. Now, if, after lightly sanding the primer you are satisfied with the primer (not too many bald spots), you are ready to start with the paint. If, however, there are too many bald spots, apply another coat of primer. NOTE 2: From here on you will lightly sand between coats. This is very important because the sanding between coats is what gives you the good, smooth finish you are looking for. So, you will keep building up coats of paint until you are satisfied. I would recommend at least 3 coats of paint, not counting the coats of primer. Try not to sand after the final coat of paint. Final Thoughts: I would recommend using spray paint for this job. It puts on thinner coats, but the final product will look better than paint that’s brushed on. TAKE YOUR TIME! Don’t rush the job, make sure the primate and paint are completely dry before sanding. You will get much better results if you paint with the frame laying down. Don’t spray on too much paint in an effort to completely cover the entire frame in one coat; build your coats slowly. If you make any mistakes along the way, such as runs in the paint, just grab for the sandpaper and sand the imperfection out. If you follow these steps you will obtain the finish you’re looking for. I have been doing finish work on the cabinets and furniture I build for more than 25 years, and I am glad to tell you what I have learned along the way. Best of luck.

I use rustoleum spray paint...lots of light coats and the results are flawlessly smooth !!