You should be asking the question about sanding sealers. A lot of folks use them as a sealer, and often as a finish, because they are a shortcut to a smooth wood surface. However, we should always take a look at what we are putting on a piece of wood, because there are always sacrifices being made for that "quick" finish.
The commercial "Sanding Sealer" is common shellac or nitrocellulose lacquer with something added to it to fill the the grain of the wood and act as a lubricant for sanding.
In the old days that added "something" was a wood flour, which is wood dust with the consistency of flour. We made our own by adding sanding dust to whatever finish we were going to use on the wood. It did a good job of filling, but didn't do anything for sanding.
That "something" in modern sanding sealers is a stearate. They are used because in addition to filling the grain of the wood, they provide a lubrication for sanding.
Stearates are an animal product and are more commonly found as an ingredient in soaps. You can see calcium stearate as the "soap scum" that is formed on the surface of hard water.
It is opaque, as can be seen in the can, and while it is filling the grain of the wood, it is also masking the color and brightness of the wood. Used in excess, it will cover the grain of the wood with a white coating. Whether there is any effect on the adhesion of any finish that is put over it is still an unsettled argument.
I don't use sanding sealers because I am trying to use a finish to accent and brighten the grain of the wood, not mask it. Since I don't use sanding sealers, I have no experience with their effect on other finishes that are put over them.
I think there are better ways to fill the wood grain on a pen, but they do take longer. CA can be used as a filler that is clear, it has the added benefit of hardening the wood, and there is no problem with any finish sticking to a sanded CA surface. The finish itself makes an effective sealer, but it will take more pains with the sanding because we may have to use wax or water as a lubricant, and it will take a couple more coats of a finish to fill the wood grain. The results are always worth the effort.