I advise you not to sand any oil paint. The dust can be highly toxic and even deadly.

Stand oil is great for reducing brush marks. You’ll still get ridges in your paint if your not careful but it reduces all the little fine hair marks that throw so much glare. It’s like painting with honey though.

It’s always difficult to give an advice, when you can’t see exactly what is going on. But this is mine:

(Thoroughly) sanding, I would only do when the paint is completely dry (after months/years). Very, very slightly sanding is not a problem as long if you are not damaging the protecting layer on top of the paint layer. And when you sand, it’ s best to paint over it with a fresh layer of oil paint, after you cured what you sanded it with some medium.

If you have brushmarks (relief), and the paint is to fresh, you’d better remove the paint completely.

If you want no brushmarks and and an evenly smooth layer, paint as a housepainter: different thin coats of paint. Do not try to cover it with one(thick) layer.

If you knew it in advance, you could have applied an thin acrylic layer first, before starting in oils. Acrylics are more opaque than oils.

You say that feathering the surface might work. Well do it. After feathering, you”ll have to remove the paint which is distributed on the figure (try with a nearly dry brush sparely drenched in medium/OMS).
After that, you can still apply a thin layer of the same colour.

Here you go, this is a [URL=https://youtu.be/KRRZLVu-IQI?t=1020]time stamp[/URL] from a video I watched not too long ago. This is just for shaving the initial bumps out.

Then I have another [URL=https://youtu.be/KRRZLVu-IQI?t=3840]here[/URL] where he proceeds to sand some of those areas the rest of the way down and talks a bit about what and why. Watching the whole video was interesting as well, and there was a lot to learn from it.

No offense intended, but boy, that video was boring.

I actually had some success removing brushmarks that were in the wrong place (in the background, competing with the subject) in a completely dry painting. I used a very fine wet-dry sandpaper. If the painting is still wet, there’s a tool I used to have that would have done the trick perfectly. It was a ceramics tool that had a long needle point on one end, but on the other end was a small metal claw-shaped (as in single talon) piece that had fairly sharp edges and so many uses, including scraping back wet or even dry paint layers, and loosening dried paint from the inside of a paint tube spout (is there another word for that?), and also for loosening the dried paint from around a stuck cap. The pointed end works great for cleaning out the threads on a dirty cap.

Unfortunately, that piece on my tool came loose and, unbeknownst to me, fell off and was probably vacuumed up. I’ve searched all over for that same tool and hadn’t been able to find one – until now! This looks like the one I had: http://www.bigceramicstore.com/6-5-kemper-lace-sgraffito-tool-k25-20341.html
And so does this one: http://www.bigceramicstore.com/6-75-kemper-lace-sgraffito-tool-ab.html
There are several other variations on a “lace tool” on the website, here: http://www.bigceramicstore.com/tools-supplies/decorating-tools/sgraffito-tools.html

I’m going to have to study the lace tools more closely to see which is closest to the tool I had. Basically, FYI, they describe the end that got lost as a “curved, pointed blade.” So, anyway, thanks so much for asking the question! I might not have found this otherwise. Also, I’d try sandpaper first, as you need to be extremely careful using the claw blade on a canvas surface.

Also, I’m pretty sure the only danger with sanding would come if toxic paints, such as cadmiums, were used. Other paint ingredients are typically benign.