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I've picked up 2 old bowlback mandolins at garage sales. Both are really dry and in need of repairs. I think they need to be rehydrated. But I've never done that before so I guess I'm looking for the best way to do it. One even had mud dobbers nests in it. Any help would be appreciated.

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Many of the bowl back mandos were part of a time when ornamental, import instruments flooded America and were intended to be basically that, ornamental. They do not make decent musical instruments and are often as they were when they were new if you restore them, fraught with issues when it comes to be actually playable. They were intended to be hung on the wall.

This is not all the bowl backs mind you but likely most of them. They were assembled with very thin woods and their value proposition was often how ornamental the thin inlays are. These days when we see them we won't work on them and usually turn them away because they never were intended to not only be a real, playable instrument but also they were not intended to be serviceable. Fix one thing two others break.... Full string tension on these can be interesting too, keep your face back.... That is if the tuners still work.

The typical way to rehumidify an actual wooden musical instrument is to place it in a place with controlled, higher humidity. We bag instruments with car wash sponges and some water in the bottom of the bag never touching the instrument. we've measured inside the bags at around 75%. Several days at this RH usually brings back a guitar or Mando and we've never observed any other casualties from the process. I would not go more than five days though with this method and would not do it at all with these bowl backs.

Reason being it's likely that some of the inferior glue joints will fail and the inlays will come out as well as structures may lose their glue joints.

Again these were never intended to be serviceable making them usually not worth any effort to restore unless you want them to look better hanging on a wall.

Lastly there were some that were intended to play but they are not as common and there is no easy way to tell what you have.

If your goal is to learn about musical instrument/mando construction and have some knowledge that will be useful again in your Lutherie journey find an old Harmony Mando, A style. Infinitely repairable, very playable and I have one that I restored and it's around 60 years old now and plays and sounds great. Old G*bson A styles are great for this too but much more pricey and also much more in demand. These often start at around noon $1,700 once restored and playable.