I find if you use it in a caulking gun with a small opening you can run along with the gun at 30 degree angle laying back while pushing forward you can really get it in there. Working quickly lightly spray prepsol on it, then quickly remove as much as possible with your finger (spray prepsol on it first). You cant have gaps in the caulking before spraying the prepsol otherwise the next run of caulking wont stick. don't fiddle with it when its starts skinning over you'll just fudge it up. Practice around the house with a garden sprayer and some silicone (Kitchen etc) that'll give you the feel for it. the seems on my van were wire wheeled, rust converted, then primed before the caulking was put in.

If you are not confident with the finger then fold a piece of new clean kitchen towel (the blue stuff) for the final pass.

Be organised and work as cleanly as possible.

If you fudge it then you can practice again! LOL

I'm no expert but i hope this helps.