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Captain Jack Sparrow

Vacuum Bagging wet core area to pull moisture out?

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May be doing a balsa core deck repair in the near future. I figured I would cut the top skin off, rip all the wet core out in every direction until I come across dry core, and be good to go. It was suggested to me to hook it up to a vacuum bag for a few days or whatever in order to pull any additional moisture out of the surrounding core. 

Is that really necessary? I can see why the concept would seem like a good idea, but if I get to dry core material on all edges, it shouldn't really be pulling much moisture out if any,

 

I have access to the equipment to do it and am not at all opposed to doing this step.  Just wondering if other people who have done core repair jobs think if it is worth it to do. 

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If you have any tell tale signs as you do the job and you have the kit I would definitely do it. Especially if you’re vaccing the new laminate back down, nothing worse than pulling a Vac and seeing little spots of water pulling up through the new laminate!

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Another option is while the deck skin is removed and you've already ablated the wet dead core, you can tent the affected area with plastic sheeting and put a dehumidifier on full blast for a few days . That should pull any remaining moisture out too and save you the hassle and materials of doing a full vac bag -

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13 minutes ago, ScowVegas said:

Another option is while the deck skin is removed and you've already ablated the wet dead core, you can tent the affected area with plastic sheeting and put a dehumidifier on full blast for a few days . That should pull any remaining moisture out too and save you the hassle and materials of doing a full vac bag -

I've got the materials and equipment, no worries. I think I could set it up with minimal effort. I just didn't know if any significant migration of moisture under vacuum happens through balsa in the span of a few days. It sounds like it make sense to do just as additional insurance.

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Two types of moisture in any material: surface moisture and "bound" moisture.

Vac bagging will drive off the surface moisture quickly and easily, say overnight. Its presence will inhibit subsequent adhesive bonding. (Remember to ballast your vacuum pump.)

Bound moisture diffuses out over a much longer time, it can be helped along by heat but it's a slow process - not worth the effort (probably not worth starting if the core is that saturated.)

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9 hours ago, Captain Jack Sparrow said:

I've got the materials and equipment, no worries. I think I could set it up with minimal effort. 1)  I just didn't know if any significant migration of moisture under vacuum happens through balsa in the span of a few days. 2) It sounds like it make sense to do just as additional insurance.

You are right on both accounts: 1) balsa - in contrast to foam - moisture doesn't penetrate much. It is often possible to see a rather sharp front where the balsa is gone and where it still is fresh.  That's the advantage of balsa, downside is it rots if wet.   But if you have all equipment and can easily set it up then 2) by all means do it. Some extra insurance is never wrong. 

Most important is to remove all damaged parts. 

 

//J

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16 hours ago, Fleetwood said:

Two types of moisture in any material: surface moisture and "bound" moisture.

Vac bagging will drive off the surface moisture quickly and easily, say overnight. Its presence will inhibit subsequent adhesive bonding. (Remember to ballast your vacuum pump.)

Bound moisture diffuses out over a much longer time, it can be helped along by heat but it's a slow process - not worth the effort (probably not worth starting if the core is that saturated.)

^^ this. The vacuum required to vaporize water at normal temperatures is quite high, higher than you will get with a vacuum bag on a portion of hull. Look at the vapor pressure of water at various conditions for proof. You need low relative humidity and air movement to pick up and transport the water outside. The low humidity can be arranged with a tent and dehumidifier, the transport is more difficult. If you were worried about the margins, you could dehumidify the area, vacuum bag the edge of the hole, then drill some vent holes back aways from the periphery - this will allow some dehumidified air to leak through to the vacuum, which WILL transport the moisture out. It still takes time though. With enough time, you dry a boat completely out this way. 

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There are really three stages of wet core with balsa, the first is where it is basically reduced to compost which is obvious but as you keep removing skin you encounter the next stage which has good color and looks healthy but you can dig it out and it is softish and you can squeeze water out of it so you keep going until you encounter nice healthy balsa that is as hard as fresh stuff and it feels dry to the touch, the skin is tenatiously bonded so you stop there. The problem is the moisture meter is still telling you its wet beyond where you stopped . If you were to leave the core alone at this stage but continue to remove the skin it would dry out quickly with exposure to air and particularly airflow but if you don't it will remain showing wet forever so that even if you put in all this work someone with a moisture meter will try to chew you down when you go to sell off on the future. I always seal the edges of the old core when installing the new but have never tried bagging the area  back where the scarf will be and beyond so will be interested to see how you get on and if it achieves the desired result. I agree with DDW that you will need holes beyond the bag to allow the vacuum to transport the moisture away via the pump but that means your pump will be operating under rough vacuum so you need a pump capable of this for however long it takes, so a proper industrial pump, not a little ac evacuation pump.

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+1; you'll need a proper vacuum pump, continuous duty rated, preferably a (2-stage) rotary vane pump. It's not the capacity or ultimate vacuum that's needed - you are pumping water after all - but continuous rating.

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I'm not sure you need to worry that much about the pump. A normal Gast carbon vane pump will work fine. If you drill the vent holes I'm talking about, they can be small (1/4"), and a moisture meter can be your guide. It doesn't really matter that the pump can't pull a vacuum on the holey boat, if fact if it can't so much the better - that means more air flow is going through. If you can get air (dry air!) to go through, it dries quite quickly. The rate is highly dependent on how much air you can get to go through. You can guide the air to some extent by taping up holes, forcing more flow in areas that still read wet on the meter. You are going to have to run it awhile - few days to a few months depending on the extent of the problem. 

The flow is important, the vacuum reading on the gage not very important. You need to get an absolute pressure close to the partial pressure of water at normal temperatures for the vacuum to do anything, that's getting near deep space type vacuum. Go for flow. 

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Nonsense! If you can get a good seal then with a vacuum pump you will be quickly only pumping water vapour. Its rate of removal is governed by surface type and area, temperature and sizes of gaps. Pumping air through will just exercise the pump a bit more. Again, surface water should evacuate in several hours.

At 20c water vapour pressure is about 0,02 atm, the sweet spot for vane pumps which pump to about 0.001 atm. (Deep space is less than 10e-14 atm.)

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Even Gast doesn't claim 0.001 atm in perfect lab conditions. That's 29.81" mercury. In a vacuum bag on a boat (which is going to have many leaks) a more typical value is 20-22". If you have a really good bag and an otherwise well sealed laminate maybe 24. From the Gast catalog, 28" is the best pump they make, and again that is with the intake sealed. 0.02 atm is 29.3". Good luck with that. 

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Done it more than once. Vacuum reduces the vapor pressure, so whatever water there is will evaporate faster.  Use heat as well.

Not  uncommon to boil water out of balsa under a vacuum bag if it hasnt been dried before laminating.   Painful lesson that one

You  will want to put a trap in the line between the pump and the part, because somehow there always is some condensation and it's better if your pump doesn't eat it.  It alway takes a long time to dry things out, and you really can't rush it, so if you have a tight deadline, just cut until you get to dryness.

SHC

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Wasn't familiar with Gast pumps, which look as though they are more process gas pumps than high vacuum pumps.( I use an old Edwards 2-stage vane pump ( salvaged from an earlier life in high vacuum technology) for my vac bagging, easily gets to 0.01 atm, open the ballast to keep water out of the oil. Trap is a good idea to keep epoxy out of the pump tho'!)

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