Aside - Try rolling and laying off with a brush to get a smoother finish than just brushing. Test the roller first though to make sure it survives the two-pack solvents.
Re: Paint Transition
Tape off on the existing painted deck say an inch from the repair, lightly wet sand over the existing painted area upto the tape, (re-tape if necessary), paint the repair up to the tape and remove the tape with the paint still wet, wet sand and buff the transition area.
If you want to experiment and the job is small enough, you can try a Preval re-usable spray can...
Preval...available from Bunnings. Two-pack will require thinning.