Treating wood

[Mojooo]

I have some old wooden floor joists - they are about 50 years old - rock solid but not treated.

Rather than chuck them I want to use them effectivley as railway sleepers and build a small wall to sit on in my garden.

I suspect I will have to sand the visible surfaces down so will have 'fresh' wood on the surface.

Question is what do I treat them with?

There seem to be a lot of products on the market. I want them to have a dark brown finish ideally but don't want a nasty smelly surface. I coudl use treatment and then paint on top if needed.

I don't mind if they last a few years and rot but anything to preserve them a couple of years will do.