A lot of these doors were made in volume by big mills in Canada between WWI and WWII using timbers such a spruce and Douglas fir (hence the striking grain).
As a mass market product these doors were generally sold unfinished, which means the house builder would have been responsible for finishing them. I think most of them would originally have either been either painted or stained and lacquered (stuff like Sadolin oil-based varnish/lacquer). AFAIK waxing wasn't used in the 30s, and you rarely see French polishing (i.e shellac-based finishes) in domestic woodwork other than for individual pieces of furniture after WWI, which isn't surprising when you consider the amount of time and physical labour that goes into French polishing, even when you cheat and lay down three or four of the coats by spraying. To my mind it suggests that you either have a sun-shade type marking or that someone may have tried to strip and wax the door at some time in the past - and wax can be a bugger to lift out of grain. What effect does sanding have, if any?
I'd caution against using any form of metal brush on a relatively soft softwood such as this - you can use soft brass brushes on some open pore hardwoods such as oak and ash, but they are physically a lot harder than Douglas fir and they have open pores which csn accumulste muck and wax, but firs lack these pores. TBH I think any form of wire brush on this door is risking damaging the surface and that solvents such as methylated spirits or white spirits with copious amounts of cotton rag are a far safer approach
.
