Select one of your finest new paint brushes. Dip it into the Tenax, then touch the brush to the seam. Repeat along the seam (dip touch, dip touch dip, touch) until you get it closed. DO NOT get into the habit of 'painting' it along the seam, once the plastic softens you'll leave brush strokes. The brush is nothing more than a transport vehicle to carry the Tenax to the subject allowing a somewhat precision application process. The Tenax will transfer from the brush to the model and flow along the seam through capillary action. Pinch the seam as you work along it.
Practice using a few old fuel tanks or bombs to get the feel of it. Use the brush only for Tenax nothing else. Tenax evaporates rapidly so there is no need to 'wash' it afterwards. Should it get a styrene crust, wash in lacquer thinner. Store your brush on the bottle, hold in place with a rubber band bristles up.
I believe Tenax has an applicator brush in the lid...I usually just remove and toss it away because it is too fat for fine work.