Hi!
You don't sand the seams when using Tex coverings,at least it does not work good, but naturally it could be done! If you want them to be unseen paint the wing using automotive 2 part laquer paint!
But please do not use a small air brusch, they are just for small details, not for a large area work like on an R/C plane. For painting an R/C plane use a large automotive spray gun with an above cup (3/4 liter).

But I would not recommend using TEX coverings on ordinary sport airplanes as these coverings are heavier than Oracover (which is the best plastic covering).
TEX covering does not have to be painted but remember that they get dirty with time from oil residue.