George wrote:

Just like the warning on your lawnmower to keep your fingers out of the
blade, the warning is given because some nimrod is likely to use a stearated
sanding sealer and then complain. If you use a sealer, versus a sanding
sealer, which is normally stearated, and even feels a little soapy, no
problem.

Sanding after staining can really mess the surface. I'd rather scuff the
first coat of dilute poly, which can be used as a sealer.

As George says, a diluted coat of the finish itself can be used as an
excellent sealer, if time is not a factor.

The OP should ask him or herself WHY a sealer?

Sanding sealers are typically used to speed the sanding / finish
building process, before more expensive, less sandable, and sometimes
much slower drying finish coats are applied. A good sealer can often be
sanded within the hour.

If the final goal is an open-pored look on red oak, a sealer may not be
necessary at all, or the diluted finish might be perfect. If I'm
looking for a bowling alley mirrored surface, I might take most of the
early coats of real sanding sealer off with 150 or 220 grit, before
ending with a 320 or 400 grit scuff of the last coat. Genuine sealers
let me do this in one day, diluted poly takes a lot longer, as each
successive coat needs a lot more time to sand without "pilling".

I've found that the better sealers will mention right on the label if
they are usable under polyurethane. For instance, Zinsser Quick 15
states right there on the label that it's stearated and lists the
products it's intended to be used under. I'm pretty sure it even says
"not for polyurethane" in bold face print.

My absolute favorite, truly universal, sanding sealer is Zinsser Seal
Coat, which happens to be a simple premixed 2 lb. cut of dewaxed
shellac. Seal Coat works GREAT under any finish where you'd use a
sealer, and is well worth the time to learn to use. Learn it, and
stock ONE product. It can be sprayed right out of the can, applied with
a foam brush or rubbing pad, or a good quality paint brush.

I've dedicated several brushes to shellac (and Seal Coat). When I'm
done using them, I let them dry. Before the next use, I drop them in
alcohol, and in 15 minutes they're ready to go.

In most cases, with the exception of Seal Coat, I try to stick with one
brand of finishing product on one project. For example, if I'm using
Behlen (Mohawk) lacquer, whenever possible, I'll use their toners and
stains. If I'm applying a Waterlox, Pratt & Lambert, Sherwin Williams,
etc... varnish over someone else's stain, I'll make sure there's a
barrier coat of Seal Coat between them.

As usual, PRACTICE on SCRAP! G

Any time you're unfamiliar with a process, keep a test board (of decent
size, NOT 3"x4" G) available, do _every_ step to the test control,
including sanding, and write the steps and notes on the back. ONLY when
the step is successful on the test board should you do it on the
project. Great finish? Save the board so you remember the steps next
time, and to show the "customer" (even if it's yourself and the wife)
what the finish will look like.

A good finish really makes your great work look as great as it really
is, don't rush it. Take the time to really get to know what you're
doing. Nowadays, I typically consider a project about half done when I
start finishing. As a beginner, I was almost done!

Have fun!