In the videos where they're applying the liquid cement to the outside of a joint, they're probably using Tamiya Extra Thin or something similar. It does melt plastic, which is what makes that technique so effective. What I do with Tamiya Extra Thin is to put the two pieces I want to join together first, and then open the joint just a tiny bit so there is a gap between them that is no wider than the thickness of a piece of paper. This provides an area for the Tamiya Extra Thin to go, and it keeps it from evaporating instantly which gives it time to work on the plastic. You don't really want to "run a bead" though, you just lightly touch the brush to the joint and you'll see the cement get drawn along the joint by capillary action. I usually do an inch or two at a time. With each tap of the brush on the tiny gap between the pieces, I then wait about 20 seconds before firmly pressing the pieces together. You'll see a tiny bead of melted plastic get pushed up from the joint as the two pieces are welded together by the solvents in the cement. At the same time, you want to check the alignment of your two pieces to make sure you get panel lines aligned, and also check the up and down alignment of the joint so the two sides are flush with each other and you don't have a step. You can make adjustments to the alignment before the plastic solidifies again. 24 hours after gluing the pieces together, I use MicroMesh sanding sticks, starting with 1500 grit, and gently sand away the bead of melted plastic, which leaves the pieces welded together, with virtually no visible seam between them.