Bushing Refurbishment: Bushing refurbishment can offer a short term while waiting for replacement bushings or long term solution to circuit down time.
During the original manufacture bushing cores are wound dry:
Resin impregnated cores - cores are placed in a processing tank and liquid resin drawn into the heated paper core under vacuum
Resin bonded cores - cores are wound on heated rollers with resin coated paper
Barrier cored bushings are built with a series of resin bonded paper barriers 
Oil impregnated paper cores - cores are dry wound paper with foil capacitance layers, the core is then placed in a processing tank with other bushing cores, the processing tank is sealed, vacuum applied and the tank heated to dry the bushing cores, then the tank is filled with clean bushing oil and cores allowed to soak, the tank is then drained and the cores are removed and stored until they are required for bushing assembly, after assembly the bushings are electrically tested to prove the bushing core.
When refurbishing the bushings they are being rebuilt from this stage using a previously tested and viable bushing core they are heated under vacuum and filled with clean dry oil, in our experience most bushing failure is caused be 
1. Poor Oil - natural aging and moisture build up causing the generation of gasses through category 1 then category 2 and finally category 3 leading to breakdown of the bushing core layers
2. loose internal connection due to vibration and thermal activity during the bushings service causing hot spots possibly arcing and gas generation as above
3. test tapping internal wire damage or arcing in the test tapping housing causing arcing and gas generation  

If refurbished before the internal gasses and breakdown of the oil reach high levels, causing arcing between the capacitance layers of the core then the bushing can often be returned to service.
often 10kV on site testing can be sufficient testing as the original core has been previouslyproven 
we have previously refurbished a bushing with >5000 parts Acetylene, the gas generation had been caused by poor internal connections causing arcing, when refurbished the bushing passed full electrical test


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