Difficult windows need tidy join

[WorkingHard]

Hi. I'm trying to deal with my 1930s Crittall windows.

The interior side of the windows needed repainting as they were flaking away and rusting on the inside, especially on the oriel window.
I've stripped paint off, sanded, done the rust treatment and done my best to even up the patchy filler at the bottom of the window to fill in holes.
I've used a zinc metal primer and put on my first coat of gloss. (It needs one more coat).

The problem I now have is that trying to get a straight paint line along the bottom where the sill is is impossible. I need to cover up the filler areas as they are a bit uneven in surface texture and colour, (the repair is in the middle with older filler still working but darker either side). I am fully sealed now. The paint isn't going on smoothly enough to make it look nice. (Some surfaces are smooth whilst others are bumpy.)
I'm thinking a line of sealant/caulk squeezed out in a line would do the trick after painting to make it very neat.
I'm using white gloss and the sill is unglazed terracotta tiling.

I'm not sure whether to look at decorator's caulk or another type of sealant. I've read that silicone is difficult to use for amateur use. The window fittings that are part of the frame make it difficult to access some small areas.
I also plan to clean the terracotta tiling and maybe wax protect it in the future so it simply doesn't pick up marks so easily. (I would love to have some plants on the sill.)

So, what sealant/caulk would be best and most friendly?
Should I paint first or afterwards? I'm thinking afterwards because this treatment is purely cosmetic to give a nice white line along the bottom so the sealant/caulk should be white.

Thank you all in advance.
:)

[chris_on_tour2002]

decorator's caulk should be fine, don't use silicone if you want to overpaint it.

white silicone will work though it can discolour over time if it gets dirty and can go mouldy. wouldn't advise it if it's not a product that you are familiar with.

use masking tape on the sill to give a clean line for caulking and painting. carefully peel away once dry.

[shedjerry]

Having just done a similar job, silicon is good if you don't want to paint, like chris said, but I wouldn't recommend it outdoors because it goes bad fast.

Caulk is what I'd recommend!

[welsh brickie]

why not use white, small quadrant, plastic trim you will get a much neater job.