Doors and Windows - Exterior caulking

Installed replacement Milgard windows. They look great but would look much better if I actually finished the job. I have put up 1 x 4's around the exterior of the windows. I used a mitred corner for a littler nicer look. That was my downfall. Not being perfect, or even a professional at this, there are small gaps where the corners should meet. I would like to fill the gaps, which are small, and paint over it to hide the flaws. Can I use regular old joint compound used for sheetrock (easy to work with) or is there something more appropriate?

Phenoseal will work well...........Also make sure you put a nail in from the top side which will help keep the corners together.