This is used with water slide clear decals. It helps adhere the decal to the object so air isn't trapped under it. There's also a decal softener that can be used to help the decal film relax and take the shape of the surface. It could be helpful if you've never applied waterslide decals. It helps the clear decal paper edges blend in by eliminating air being trapped under the decal but I've never used it on pens since the surface is so small and smooth. If your applying on a smooth surface it isn't a big issue as long as you apply the decal to a small puddle of water on the surface and then brush the decal from its center outwards to remove the water and air under it. Most hobby shops will have some brand of these.