Of need of caulking help

[Customride]

I am building a 17 foot twin cockpit runabout. Where I’m stuck is caulking the decking. It is Mahogany planked that I have stained and have eight coats of varnish on it. Now I started to caulk it and two things are happening. The first thing is it is shrinking a lot and second is when I pull the tape off (caulk is Still soft) it is leaving little peeks and it’s rough. How do you get those beautiful y smooth white caulk line without messing up that nice varnish job??????
What type of caulk works the best and how do you do it?