I assume you paint only on a Polygon Object, but the image shows the Symmetry Object. The manual does not state the Polygon base as a requirement here, but I run into problems recreating your set up. As usual, I tried it and do not answer from memory.
The image does not answer if you have tried to “paint” on the object close to the border and where things are more tangential than perpendicular. Try to work in a way that the surface is right in front of you.
Since I write in a forum, I have to point his source out:
Your question indicates more a “fix it in post” workflow. Let’s try to do it in a useful way from the start. Keep it centered and perpendicular.
There are no tools that would have the same effect while smoothing it while acknowledging the surface. The ShrinkWarp has three modes, which are often not useful for more complex objects.
(I think it is a great idea)
I typically use the Shrink-Wrap, with a Restriction Tag and paint via Vertex Map the surface into place, but that is not what you exactly asked for. I made some screen capture with this technique; let me know if there is any interest.
Or simpler not smoothing
Perhaps you try to use a completely different alternative:
Which is certainly faster and with the Spline direction, hopefully working towards your needs.
I tested the setup in conjunction with the Symmetry Tool; again, I do not answer from memory, as things might have changed. Here is can read the information, even previously generated.
Please provide, if possible, a reduced scene-file. Images have very little value compared to this. It has shown over and over again that scene files communicate better.
If the attachment options don’t work (perhaps size-wise), I can provide an upload link. I touch only Dropbox, Amazon, Apple, Google, Adobe or Wetransfer, never abbreviated links (e.g., bit/ly), of course, but who would.
All the best