To the left is where another company tried doing a test spot for paint stripping and failed. The reason? The first layer is milk paint, thicker than all other coats combined. In the early 1900's people mixed glue and paint together thinking it would help bind the paint better. It did, but it always stays like hardened tar.

You can see that PPWP knew what to do below.

13-15 coats of paint were removed to the bone & fresh coats of solid stain were applied.

Not many companies have anywhere near this type of experience. 

Full interior paint stripping job explanation on the Interior painting page.