What is this old paint?

[inventor]

I'm working on some exterior softwood mouldings from a house built about 1840. I think the mouldings are original. I stripped off many layers of paint with Nitromors, but what seems to be the very first coat is different. Nitromors doesn't touch it at all, no matter how long left. It's a very hard finish, more like a ceramic than a paint. I can just about remove it with a scraper or a power sander. It's very light brown.

It's not really a problem - I'll leave this finish on, mostly - but I'm curious as to what it is, and why it's resistant to stripping. Any ideas? Thanks!

 

[Scrit]

Having been experimenting with this stuff recently I wonder if you might have found a layer of casein paint, otherwise called "milk paint". Unlike oil-based paints milk paint seems no to be susceptible to conventional paint strippers, but I've found it will come off if you apply a strong mixture of lye (sodium hydroxide). You want a mixture with the consistency of a thick paste and remember to add to lye to the water not the other way round. If you use lye you'll need strong rubber gloves, an impermeable apron (PVC kitchen apron will do) and goggles as it is not healthy stuff to be around. Skin contact must be washed off immediately with water. One supplier of milk paint sells a remover which is a combination of lye and soda ash (washing soda) so I suppose that some washing soda added to the lye must reduce surface tension and speed up the stripping process. If the timber darkens then application of an oxallic acid solution should restore the colour. After using these materials the panels need to be washed down thoroughly with water and dried.

If the house is of architectural merit I'd check with the local Building Conservation Officer at your local council before proceding.

Scrit