Nothing here is set in stone. We only offer our experiences to those who have not done this before to use as a baseline or starting point. I have used Eternabond tape on newer trailers and it does work. That stuff is about the stickiest thing I have ever worked with and when used properly can be a real life saver. When economics or lack of time dictate, we must use what we have or what we can get a hold of. It'a all a matter of balance.
I Gutter Seal *neatly* by using paint thinner. You can smooth with a wetted finger and remove excess as well. It's not perfect but clear is hard to see. I just did a tiny Hehr window tonight that the PO annihilated. Looks almost OK.
Like Vikx said, I used my finger to apply about 36'' at a time, then quickly wiped with solvent on a rag (turpentine). You have to be quick 'cause the stuff sets up FAST and once dry is really hard to remove. The nice thing about it is you can paint over it, so if you are applying it before any painting has been done it will hardly be visible. I don't think I'd put it on over paint though.