Hi guys

I am replacing a garage door and wanted to paint over the door trim.

See pic below, currently there is a 3-4mm gap between the two pieces of plywood (there is also a larger gap at the top but im going to cut a piece of plywood to fill in and paint over)

I understand I would need to sand the whole thing down before applying a sealer/gap fillter then paint it over?

The objective is to make the whole thing look like one piece.

Is Bostik 430g White MS Safe Seal my best option for filling the gap?

https://www.bunnings.co.nz/bostik-430g-white-ms-safe-seal_p0098516

Appreciate any help.