roof rain gutter finishing?

Yes, you need a seam sealer in the roof rail gutter. Folks use different products including those from the hardware store not intended for that job. The choice is yours but from experience I'd recommend using a two part product such as Lord Fusor 123EZ or 129. It does require a special gun to dispense but it self levels, can be tooled with a finger, and cures in minutes.

Mask off where you don't want sealer, apply it, smooth it, pull the tape right away and you're done.