Why does caulk/solvent based paint remain tacky and yellowing?

If your sealant or silicone has gone yellow or is in the process of going yellow, don't worry. It is a common occurrence. Your sealant is likely going yellow because of exposure to UV radiation

Over time the snakelike in the solvent oxidises

All solvent paints no matter what the manufacture may say they become oxidised,
Which results in the white paint turning yellow, it cannot be stop and personally you are better off using water based as it last a lot longer, it’s not as shinny as you may say once completed but saves you having to paint the areas on a less often case
Hope this helps

Damp may be coming in from outside wall.

The walls are dried properly and that's why the yellow color comming on the Caulk.

you had to let it dry well and then go over the upholstery

If your Caulk is sticky then it seems like your not giving it enough time to dry.
Try leaving it for 24 hrs before painting.

You can re paint it again and wait until it dries.

It sounds like you may be experiencing issues with moisture or contamination affecting the paint drying process. Here are some possible reasons why the paint remains tacky and is yellowing:

1. **Moisture Contamination:** Moisture from the wall or surrounding area could be affecting the drying process of the paint. This can result in the paint remaining tacky and not fully drying. Ensure that the wall and skirting are completely dry before painting.

2. **Residual Wallpaper Paste:** If excess wallpaper paste was not completely removed from the wall before painting, it can interfere with the adhesion and drying of the paint. Make sure to thoroughly clean the surface and remove any residual paste before painting.

3. **Incompatibility of Paints:** Using different types of paints or incompatible products can sometimes lead to issues with drying and adhesion. Ensure that the undercoat and topcoat paints are compatible with each other and suitable for the surface being painted.

4. **Poor Ventilation:** Inadequate ventilation in the room can prolong the drying process of the paint. Make sure the room is well-ventilated to allow proper airflow and drying of the paint.

5. **Improper Application:** Ensure that the paint is applied evenly and in thin coats, following the manufacturer's instructions for drying times and application techniques.

6. **Priming the Skirting:** If the skirting was not primed before painting, it may be absorbing moisture from the wall, causing the paint to remain tacky. Consider using a suitable primer on the skirting before applying the topcoat.

7. **Check for Mold or Mildew:** In some cases, moisture issues can lead to the growth of mold or mildew behind the paint, which can affect its drying and adhesion. Inspect the area for any signs of mold or mildew and address the issue accordingly.

If you've already stripped and repainted the affected area with no improvement, it may be helpful to consult with a professional painter or contractor to assess the situation and provide recommendations for resolving the issue. They can help identify the underlying cause and suggest appropriate solutions to ensure the paint dries properly and maintains its finish.