I just had a soapstone counter installed and they sealed it before they left. From what I've read, soapstone is oiled or waxed, not sealed. Does the sealer need to be removed if I want to use mineral oil to get an even patina?
A sealer might have been used because there are several types on the market. However, the sealer will dry in a few days. If you want to be sure of what you have just start all over by removing the finish. Most manufacturers recommend using #80 grit sandpaper to finish soapstone. I would use a random-orbit sander. You don't want to use an aluminum oxide sanding disc because it will not cut into the soapstone very well. I use the DuraDisc on hard surfaces because they work really well and last a very long time. You can finish the counter top with a 120 grit paper by hand in order to get closer to the faucet and other tight areas you might have. A lot of people use mineral wax to finish a soapstone counter top. I use Van's Beeswax instead. It seems to last a little longer and the sheen is not a wet glossy look.
I know this post is rather old, but I thought I would add a few points as well.
Soapstone does not need to be sealed. Soapstone is, for all intents and purposes, chemically inert. This is the reason why laboratory benches historically used soapstone. Installers usually recommend a finish from #80 to #120, followed by an optional coat of mineral oil, walnut oil or some type of oil/wax mixture. You don't need to necessarily stop at #80. I've gone to #220 and it gives it a very silky smooth feel. I've heard of people going higher. The downside to this is that if all you use is mineral oil, you may have to coat it more.
You also don't need any coating at all. It's completely aesthetic. Lately, a new thing I have been trying is cleaning the surfaces with barkeepers friend. Since it's an abrasive, it removes the oils and waxes and smoothes out the countertop even more. Once dried it leave the counter smooth with a flat sheen.