AusBonsai

This tree was a lovely air layer that I was going to post to the air layer thread after I was convinced it was happy.

This and 2 other F. benjis I did big chops on late Spring were both very unhappy when I've done a successive cut late Summer. In both cases I left a few inches of space to accommodate some die-back, but both trees now have extensive die-back.

There are 2 things I'm going to have to consider going forward; I may have cut in the wrong season when sap flow is slowing down? It appears to me that benji's die-back where there is limited sap flow, die-back appears to be in sections where there are no branches. Next time I think I need to wait for more budding and then cut above branches. I'm hoping to learn something from this when I work with the species next time and share my experience. Both of these ideas are just my thinking and may be incorrect.

Here is the front where the growth is:
Here is the back where rot has completely wiped the bark off from top to bottom:
I still don't want to throw this tree away, I think the dead wood can be worked with. I want some advice on how I should go about preserving the wood so the tree doesn't just completely rot away (front side looks healthy enough)? I do want to do some carving with the wood first to pretty it up a bit, which needs to be done before preserving?

Thanks

Firstly, Ficus Benjamina is known to be one of the less resilient figs. It does not tolerate root pruning as well as PJ and can fail to bud completely when chopped to bare wood.
There may be something to the timing but more likely just the species.

Provided the tree is still growing it won't rot away completely. The section which doesn't have and live branches so no sap flow dies and that wood will rot away. The live section at the front should continue to grow and the tree will eventually develop into a hollow trunk tree.
There are several ways to preserve dead wood:
Traditionally lime sulphur is painted in the dead wood a couple of times a year. It acts as a fungicide to slow rot and thus extend the life of dead wood.
More recently some growers have been trying more modern wood preservatives like Earl's wood Hardener, available from the hardware stores. I haven't used it so can't make comment or recommendation on its effectiveness.

Is it possible to preserve 70% of what's left or do I have to let it rot out and the living part decides how much is saved (compartmentalized)?

Just wondering if lime sulfur or other product is effective on that amount of deadwood? I've never used it before.

Superglue

The living part really has very little to do with the dead part so, without preservation, all the dead section will eventually rot away. In the meantime the live section will grow larger, as it always does, so what you see now is never what you'll have in years to come.
No wood preservative I know of is 100% effective. Even treated wood will slowly decay but then all our bonsai change from year to year anyway so expecting dead sections to still look exactly the same in 20 or 50 years is probably futile. The best you can hope for is to keep the dead section for a while.
You get to decide how much of the dead wood to keep for that time. Letting it rot naturally can be hit and miss. Sometimes wood rots from the inside and before you realize the whole trunk is hollow. I would treat the whole dead section and carve out what you don't want. That way you get to choose - depending on your carving skills.
Wood preservatives are effective on any amount of wood. Paint them on the surface and they penetrate into the wood structure to prevent wood decaying organisms from entering. Lime sulphur, superglue and wood hardeners all soak into the dead wood so will penetrate to some depth into the dead section.

This tree really rotted out. I tried to get rid of whatever was boring into it, but in the end it didn't help. Half of the tree rotted away and when I went to remove all of the rotten wood there were 2 large curl grubs that had made their home inside.

A few people across the net had suggest 2 part epoxy as one possibility to preserve soft wood that was left, so that is what I attempted. I didn't realise how shiny it would look. Good thing is I doubt it will rot much further with the amount of epoxy that it is covered in

As benji's don't have very compact foliage, I'm going for a loose styling. Due to the situation with the tree and benjis not readily throwing out buds on the trunk, I think the main branch structure is what I'm stuck with. The top will eventually be lowered a bit, I am aiming for a loosely styled dome shape. At the moment it's all about the health and growth. The current styling and wire is just a general shaping, but for now it's all grow!

At the moment it's a lot of waiting to grow and then chasing buds back to increase the density and reduce the leaf size. As benji's don't bud as prolifically, I'm not aiming to grow branches too long and then cut back. Ficus do a reasonable job of thickening over time anyway, so I'm working more on density and leaf development rather than risking it growing out too far and not being able to chase buds back internally.

I am also hoping to do something about the nebari. Some smaller roots are growing forward at the base and with time these will be allowed to thicken and brought to the surface. The pot is just a grow pot for now.