For small gaps between wooden trim such as doorframes and plastered walls I use a "Flexible gap filler" - available at any DIY store. Spending a few minutes reading the containers carefully and asking store assistants will get you a product you can try.
Description from a randomly selected product
This flexible filler achieves a white crack-free fill between two surfaces where movement can occur. Controlled application is ideal for long linear gaps.
- For flexible, crack-free filling between two surfaces where movement can occur.
- Easily controlled application for long linear gaps.
- For indoor and outdoor use.
An acrylic filler for draught proofing and filling fine cracks. Can be sanded, overpainted or papered over. Solvent free formulation.
- Interior or Exterior Use: Interior
- Application Method: Knife
- Colour: White
- Size: 310 ml
- Type: Ready Mixed Filler
It is often a good idea to rake out the corner first, to remove loose or weak paint. Then sand as necessary before cleaning and filling.