I think you are confusing a lot of things. That's not a criticism because labels are often misleading or obfuscated.

Sanding sealer:
These are largely unnecessary and are likely to cause more problems than they cure. The first coat of any finish seals the wood. Sanding sealers are just a bit easier to sand. But that creates two problems: they are more susceptible to damage from impact and moisture migration than many other finishes. And second, some finishes (namely polyurethane varnish) do not stick well to some sanding sealers. I don't recommend using them.

However based on your description you may have used a "pre-conditioner" or maybe even a "spit coat" to control stain absorption. These are different products from sanding sealers.

There is, however, a de-waxed shellac product by Zinnzer called "Seal Coat" and it is labeled as a "universal sealer." This is not the same as the stearated sanding sealers that most people normally think of. This is a fine product and I use it regularly. And it makes an excellent top coat, despite the self-deprecating comments on the can.

Pound Cut:
This terminology is normally associated with Shellac (not Varnish) and indicates the equivalent number of pounds of shellac dissolved in a gallon of alcohol (its solvent). Thus a two pound cut means two pounds in a gallon, half a pound in a quart, and so on. It is not normally a measure applied to varnishes, including polyurethane.

Polyurethane:
This is an OK finish, but most people tend to over use it, read: use it on everything. It is abrasion resistant, but that means it can't really be rubbed out the way a harder finish can. It also suffers from lack of UV resistance, poorer adhesion, somewhat lack of clarity, and inability to easily repair damage. The biggest problem is that most people don't know how to apply it correctly and it ends up looking plastic. The latter problem is one that exists at the end of the brush and is easily solved by education.

When I teach people to varnish, I give several principles:
1. You have to thin it (regardless of what the can says). I thin the first two coats 50-50 with mineral spirits and thus I have a self-sealer.
2. You have to put on as little as possible consistent with complete coverage. Less is more.
3. You have to sand between coats both to remove defects and to create a mechanical bond because there is no chemical bond. You do not need to spend money on a tack cloth. Again, causes more problems than they cure.
4. A good brush ($20-30) is a lifetime investment and will dramatically improve your results.
5. Flow the finish on smoothly and slowly. When you have done an area, come back and tip-off to level and remove brush marks. Don't be slapping the brush back and forth like you are painting a barn.
6. Stir, never shake the finish before using and regularly during application. Pour off what you think you are going to need into a separate container so you don't contaminate the rest of the can. Throw away what's left. Finish is the cheapest component of the project and this is no time to save 30 cents worth of finish.

You might also consider finding some of the non-poly varnishes that don't have all the drawbacks of the $8 poly at the big box:
Pratt & Lambert #38
McCloskey's Heirloom (may now be labeled "Cabot")
Waterlox Original
Sherwin-Williams Fast Dry Varnish (not Fast-Dry Poly)

You will see many products labeled "Lacquer," "Varnish," "Urethane," "Tung Oil," and so on that are really stretching the truth about what most cognoscenti usually think of with a product of that name.

It's not to late to add the book below to your wish list. It will greatly clarify things and open up a whole new world to you.

Here is a link that might be useful: Bob Flexner's Understanding Wood Finishes

oops, You're right - I said varnish and I should have said shellac. I did use Zinnzer Seal Coat, arrgh, I could have used that for my top coat? I asked at Woodcraft for a wood conditioner and that's what the clerk recommended.
I will definitely get the book. It seems I have alot to learn!
Thanks!

Seal Coat (thinned) makes a good conditioner. This is the "spit coat" to which I alluded.

I buy it by the gallon and use it when I need to bring out a lot of the chatoyance (luster) in the wood, need to seal in silicone contamination (Pledge), seal in odors in antiques, for the interior finish (it does not smell forever like varnish or oil). I really like it a lot.

Shellac has been used for a finish for a long time. It is sort of the under appreciated finish -- it keeps taking the back seat to more modern finishes for the last 90 years. Though, as a friend of mine says, if it was new today it would be regarded as a miracle finish.

The story of shellac.

Here is a link that might be useful: wood conditioners