Is This Glue, Paint, Primer, or Something Else and How to Remove It?

Question

While using a heat gun to remove several layers of paint from a wooden door, the bottom layer or two have been particularly hard to remove. I think these layers may not be paint, and I am curious as to what these may be and how they can be removed?

The building in which I live is about 100 years old, and I assume the door, which is a bathroom door, is the same age as the building, but I really don't know. The door's top-most paint is white (see pictures below) and nicely scrapes up in strips after heating from a heat gun (1100 F). Tan in color, the bottom layer becomes gummy and sticky with the same heating and is both harder to remove from the wood and also sticks to the paint scraper.

A local paint store employee to whom I showed these same pictures thought this bottom layer was neither paint nor primer, but glue. He said that in his prior work as a painter, he discovered a decades- old painting method in which "glue paper" was applied to wood, the paper peeled to leave the glue, and then paint applied over the glue. He also said that removing this glue was, diplomatically put, laborious. I have looked for information on-line about that technique but haven't found anything.

Any thoughts on what this substance may be and how it should be removed? My goal is to remove all of the paint (and whatever else) from the door and apply a lacquer to it.

I have looked at several StackExchange posts and other sources, but I can't find any information on what this bottom layer may be.

Answer 1

I'd be doing that with paint-stripper [Nitromors or similar]. idk what that stuff is, but that's the only thing I've found that will shift it. It is still fairly resistant, but it works eventually. [Aesthetically, after stripping, I'd wax it. All that effort to then use 'varnish'? No thank you ;))

They used to use the same stuff on cast-iron fireplaces too; which would imply it's somewhat heat-resistant. That was what decided me on stripper in the first place - if it would survive on a fire back, a heat gun might not be the thing to use.

Answer 2

The layer might be hide glue, a glue made from animal hides. Diluted hide glue was used to partially seal wood before staining to prevent small regions of wood from sucking up too much stain and looking uneven. So it improved the evenness of staining.

Hide glue is water soluble, especially in hot water. You might try rubbing with a hot wet rag, or perhaps apply a wet towel and run a hot iron over it. A fabric steamer and a dry rag might work too.

Any areas that have been subjected to extreme temperatures are now polymerized or denatured hide glue and may no longer be soluble in water.