There are various products called "Sanding Sealer" with different chemical and physical properties. Not all sanding sealers are compatible with all top coats.

Personally, I don't use stearated sanding sealers because they turn white with impact and it's at the bottom of the finish. Choose the wrong product, drop a spoon on it and you'll have a nice white crescent in your countertop. It also reduces the moisture resistance.

The other question to ask is which Waterlox product are you using? Waterlox Sealer / Finish has a lower solids content (i.e., more thinner) than their Original Gloss or Satin.

I'm using the Waterlox Original Sealer / Finish (medium sheen). Would I have better results from the Waterlox Original Finish? I honestly don't remember which brand of sanding sealer he used, altho I could pick it out in a lineup! Do you have a suggestion for a sanding sealer or tell me how to tell if it's stearated? Thanks-

To cut to the chase: if you use sanding sealer, you can get away with fewer coats of w-lox, but run the risk of having the finish fail. Foolish economy, IMO.
Casey

I have had the same experience finishing a maple butcher block with Waterlox. It will bond to a shellac based sealer, but a countertop is place to avoid using sealer for the reasons stated above.

Apply more coats until the porous areas fill completely. Try applying a thin coat with a rag to the entire surface, and then follow up with a brush to apply it heavier in the porous spots. Let dry a day and sand with 150 grit between coats. Those spots will blend in after a few more coats. There is no limit to how many coats you can do, as long as you allow proper dry time. A day between coats is safe. Under the right conditions it can be sped up to 12 hours. The sandability test is best. If the finish is tacky to the paper and won't make dust, it needs more dry time.

For Waterlox and other oil-based varnishes, use black china bristle brushes only and never let water or other liquids touch the brush. Clean it completely with mineral spirits and a wire brush immediately after each use. Spin and hang to dry. You can't use disposable brushes with varnish, they'll disintegrate. And the cheaper black china bristle brushes shed a lot. Spend good money on a nice brush and take care of it.

Thank you all for your advice. I also called Waterlox and the helpful gentlemen had the same answers. Sanding sealer is not necessary or recommended. He suggested applying with a brush (thanks aidan_m) instead of a rag- thinking I was applying too thinly. So- off I go to purchase a black china bristle brush and mineral spirits... and apply more liberal coats. I knew it couldn't be that easy!