Deadwood.
I would normally start with toothbrush and plain water. Scrub the Deadwood clean. This is a good start for long term preservation, cleanliness.
Next treat with a biocide, the one most often used is lime-sulfur. Paint on a full strength coat, or a half strength coat. Try to avoid wetting living parts of the tree with the lime sulfur. Do not dilute below half strength. At some point, you drop below effective concentration, so at least half strength or don't bother. Lime sulfur will kill many wood rotting organisms, bacteria & fungi. Let the tree get sun, water, and rain for at least a month. Usually lime sulfur treatment is once a year to once every 3 years depending on how soft and rot prone the wood is.
Next step. Wood restorer. Here we add back natural resins which makes the deadwood resistant to water and resulting rots. Boiled linseed oil, diluted 50:50 with mineral spirits, will soak into the wood. Linseed oil is a natural oil, and will preserve the wood. You can also blend in some pine pitch, or pine resin. You may have to collect the pine resin from your local forest. To the 50:50 linseed oil : mineral spirits blend add no more than 10 % pine resin. In USA we can get pinion pine resin fairly easily. I don't know what is available in EU or UK. Species us not important. The pine pitch is optional, the oil of the linseed oil does the majority of the work.
Paint the deadwood with the linseed oil blend, allow 24 hours to soak in, paint again if it looks dry. Repeat, usually 3 or 4 coats will do it. When wood looks damp after 24 hours, the oil has penetrated as deep as is possible. Done for one to 2 or 3 years.
At least once a year clean the deadwood, and judge how dry it has become. Re-apply lime sulfur if black discoloration of wood rots show up. Re-apply linseed oil blend if wood looks really dry, and seems to soak up water when you are watering the trees.
Wood hardeners, are really only for soft, rotten, punky wood. Do not use super glue. In time super glue will become cloudy, and very noticeable. I am not very familiar with "good brands of wood hardeners. Others may have suggestions.
By the way, the boiled linseed oil is the base ingredient of Thompson's Water Seal, deck sealant. The good thing about making your own is it has no colorants.
Be cautious, mineral spirits are combustible, similar flash point to kerosene. No smoking while blending.
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