Best timber preservative for garden edging?

I'm thinking about using recycled red gum fence posts, resawn square to about 75x75mm to get rid of the rail notches, as garden edging / mowing strips. It's been weathered for many years and should be pretty stable, unlike some new treated timber which might warp and twist.

I think I'm better off putting the freshly sawn side on top and treating that with a standard timber protector like Bondall Just a moment...

The question is, what is the best preservative on the three sides in the ground? The timber will be at least 25 years old and heavily weathered, so probably generally not that absorbent outside whatever splits are in it.

Stick with Bondall?

Engine oil did fine on the soil side of my old hardwood sleepers many years ago.

Bituminous paint might do as well or better as it coats rather than penetrates. Just a moment...

Thanks for any opinions or practical experience.

Using timber for this puts in the maximum destruction zone from fungal attack, plenty of moisture and oxygen so you're right you need protection. I'm always leery of products like the Bondall one that claim to keep the moisture out. Timber is a straw, moisture will get in wherever it can and in my experience from years and years of doing maintenance, products that keep moisture out will also keep moisture in if it gets in somewhere else, and create rot spots. There are several Copper Napthenate products in mineral oil that provide antifungal and anti insect protection, they are usually called CN Timber Oil and you can also get it as a paste sometimes called emulsion. They work quite well. As a very effective and eco friendly alternative look up 'Shou sugi ban' You can burn the timber with a gas torch or more effectively you create a chimney out of three or four pieces and place it over a small fire until you get flame to the top (you want to suck and see on this) then quench. Edit. Often a step in shou sugi ban is cleaning off char, you don't need to do that in your case

Cheers
M

Thanks for the suggestions, mic-d.

I experimented today with charring some red gum with an LPG torch. Worked okay but given the amount of timber I need to do it'll take way too long.

The CN Timber Oil looks like the more practical option for speed of application.

Sceneys make a Bitumenous Black "paint". I have used it during the last five years so not yet long enough ago to know its effectiveness over time.

https://www.sceneys.com.au/media/pdf..._Black_SDS.PDF