A lot of deck sealers have reverted to alkyd formulations over the last few years to comply with VOC regs. These are technically oils that often don't actually strip off like oils.
Wikipedia states that "Alkyd coatings are a class of polyester coatings derived from the reaction of an alcohol and an acid or acid anhydride (hence the term alk-yd from "alcohol and acid or anhydride) and are the dominant resin or binder in most "oil-based" coatings sold to the consumer market."
Many caustic strippers struggle breaking down these alkyds. While acrylics were the nightmare of several years ago, alkyds are today's tough strips.
To make things a little more challenging, manufacturers are now combining alkyd and acrylic to make tougher deck finishes with low VOCs. These hybrid products are a horror to strip.
We asked Sherwin Williams (firstname.lastname@example.org) for their best idea for stripping alkyds and they told us that solvent (flammable) strippers such as Bix or Strypeze are their recommendation. They also suggested chemical strippers like Peel Away.
To be sure, there are super-strong caustic strippers that will break down alkyd oils - such as Rip It - at a very low cost.
We have learned that acrylics can be stripped quickly and easily using a combination of Rip It and Flood's Stripper/Cleaner. The acrylic sealer pops off in about 20 minutes. This may become the answer for the hybrid sealers as well.
NOTE: Identifying acrylics has become a lot easier these days. Wipe an isopropyl alcohol swab on the sealer. You can get these at any First Aid supply. The Sun Brite Supply store in Maryland carries them right on the counter. If the color comes off (not just the dirt) then the sealer is acrylic.
Good luck with all of your stripping challenges!