How i can delete white line?

Hi everyone !

I am not new on coreldraw, but i have a problem since few year and i never find solution

When I work, I use lines to delimit the color areas
I then use the smart surface tool to create the color box.
But that's what it gives:


A thin white line between each, is this normal and how to fix it?
(I have a solution, it is to copy paste the finalized drawing but it is an archaic method)

Thanks