Rotted Balsa core

[Guest]

I have discovered that the balsa core between the fibreglass around the fore hatch is completely rotten due to water ingress and needs to be replaced. I would like to replace this with polyethylene closed cell foam sheet (at least i think thats what it is called, swimming pool floats anyway).
Can anyone tell me what density to use in order to give the correct stiffness but bendable enough to mould to the deck curves.
I intend to carry out the repair from the underside within the cabin since there is no foof lining and will avoid spoiling the deck topside. However all the fibreglass repair books I have read talk about effecting the repair from above. I assume that this is because it is easier to work downwards from an access point of view and also to avoid fumes. CAN ANYONE ADVISE ?

<hr width=100% size=1>

 

[colvic]

When fibre glassing the underside of the wheel house roof it wasn't the fumes that was a problem but the resin dripping/running of the brush...all down arms, on your face etc..

A bit messy to say the least.


Phil

<hr width=100% size=1>

 

[graham]

I had a similar problem where water had got in around a leaky deck fitting rotting about 2 square feet of balsa core.

Like you I did not wish to damage the outside so cut away the inner layer of GRP using a strong knife and hitting the back edge with a hammer (taking care not to cut through the outer skin.

let the aea dry out after removing the wet and rotted balsa.

I used some offcuts of marine plywood to replace the core ,bonding it in with balcotan glue.Then about 3 layers of chopped strand mat wetted out and glassed over the area and onto the existing glass(the edges need to be shamfered and scratched to give a mechanical bond.

I wore latex gloves as used by mechanics ,doctors etc.And eye protection. Remove all upholstery and mask up and use old sheets etc as it WILL get messy.

I expected the gloves to dissolve on contact with resin but surprisingly they were OK.

The finish I achieved is not as good as the original.I could sand or grind it back but dont want anymore dust in the boat.GRP dust is very hazardous to health which is why I prefered to cut away with the knife and wood chisels rather than saws and grinders.You do need to rough up the jointing surfaces where old meets new grp.

Best of luck Its a good job to put behind you.Ispent more time fretting about it than actually repairing it.

<hr width=100% size=1>

 

[Guest]

Dear Colvic and Graham
I have noted your replies and noted comments for which I thank you both for your time and advice. This is only the second time I have used this forum, I think it is brilliant and you will see many more questions from me (novice sailor in the near future.
Thank you once again

STH

<hr width=100% size=1>

 

[Birdseye]

have never tried this job myself, but would suggest you consider using epoxy for the re-bonding. you can use fillers like micro baloons to make the epoxy drip free, and reputedly it sticks to old poly laminate better than new poly.

contact sp systems. they are v helpful and knowledgeable.

you could also consider vacuum bagging as a way of ensuring that the bond is properly consolidated.
<hr width=100% size=1>

 

[snowleopard]

having built 3 boats from foam sandwich i can offer a few hints-

the core provides stiffness but only if it is securely bonded to both skins so you need to stick it firmly to the underside of the cabin top. it will be very difficult to achieve that unless the surface is abraded.

nothing sticks to polythene so you wouldn't get the necessary bond. i would advise using a pvc foam like airex (obtainable from strand glass stores) . to stick it to the cabin top, a resin of paste consistency, either epoxy with microfibres or polyester 'bonding paste'.

the trickiest part will be holding the foam firmly in place while the paste sets. the most effective way is to use a vacuum bag. all you need to do is put a sheet of heavy polythene over the work and stick the edges down with e.g. duck tape, then make a hole for a vacuum cleaner hose. tape round the hose to prevent leaks and away you go.

the answer to the problem of resin dripping from the brush is to use a roller. a small lambswool (not foam) roller will not drip and will allow you to press the glass in place.

<hr width=100% size=1>

 

[steffen]

One more piece of advice. After working with epoxy resin and glass fiber i have developed a very nasty allergy. First winter made a complete rudder with out problems. Now, if i am not very carefull i end up with swollen eyes for 5 days after using the stuff. Still dont know for sure whether it is the resin or the glass, but be VERY VERY concious with eye protection.

Happy sailing, Steffen

<hr width=100% size=1>

 

[kingfisher]

Contact allergy from epoxy will occur with anyone if they work unprotected. One minute your fine, the next day you're allergic. And it's for the rest of your life. Wear protection: gloves, carbon filter face mask.

Smaller patches that need to be redone on the ceiling can be done by first applying the mat to a piece of mylar film. After sticking the lot against the ceiling, using the mylar, and letting it dry, the mylar comes off easily. But I doubt this will be usefull for a complete ceiling.

<hr width=100% size=1>Group of people on the pontoon: skipper is the one with the toolbox.
http://sirocco31.tripod.com

 

[oldsaltoz]

G'day Sirtophat,

The first thing to do is remove as much as possible from the boat and cover everything else with plastic sheets.

In the forward work area, cover the deck with plastic and any thing else that will collect dust.

When working, wear a suitable mask, gloves and eye protection, place a large fan in the companionway to blow most of the dust out the forward hatch.

Take care when removing the lower layer of glass and ensure you leave enough original material around the hatch frame to attach new glass.

If you allow the rotted balsa to dry it will come out easier, make sure you clean out all balsa at least 40 mm outside the cut line, a simple wire bent at 90 degrees in the drill chuck will do this.

Sand the ledge at the bottom and the exposed glass at the top, not a great job, a flap sander will help.

Assuming you have removed 'all' the rotted balsa it's a good idea to let it dry, you can use a heat gun or lamps to speed things up, but it must be dry. As this area has been wet, the chances of water re entry should not be ignored, for this reason you should consider using an epoxy resin as it has a higher resistants to moisture.

All materials should be closed cell, as used below the water line, this will reduce the risk of further problems for little outlay as the area is small.

Start with a thinned mixture of epoxy resin and work it into the balsa and coat the exposed glass areas, you may have to apply more than one coat to the balsa, you are now ready to apply the filler. This can be a just closed cell balloons, however I would apply a first coat of resin and fibres mixed to toothpaste consistency, just wait till the thin coat is very tacky before you apply this or balloons. Don't try to do the whole job in one coat, the weight of the mix may cause it to pull away from the upper laminate, add more as the previous layer goes off. Make sure that none of the mix with fibres in it is below the finish line, it will be hard to sand, the closed cell balloons are very easy to sand.

When all filler is in place and faired, apply your fibreglass, after it hardens, sand it smooth, and apply a couple of coats of resin, sand, and you are ready to paint.

I hope this helps. . . . .




<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[cameronke]

Gentlemen
I was thinking about a possible solution and would welcome any other criticism of my proposal. Supposing a small access hole was made into the volume and as much rotten balsa removed as possible. Now suppose that polyurethane foam was then injected into the hole (aerosol cans available from hardware stores for filling cavities in walls) and allowed to expand to fill the void. The strength of this structure comes from the fibres at either extreme (GRP) and the core is really just to keep these apart.....isn't it?

<hr width=100% size=1>

 

[snowleopard]

Every time you step on a sandwich deck the core is compressed. polyurethane foam breaks down easily (it will crush if more than 1 metre of water pressure is applied). once the core has failed there is no stiffness.

i have had problems with overloading the core of a sandwich structure, believe me, you don't want to go down that route.

<hr width=100% size=1>

 

[pvb]

Vacuum cleaners...

Remember that most vacuum cleaners rely on the air flow through the hose to cool the motor. If you seal it all up so there's no air flow, the motor will overheat and either catch fire or stop running. So you need enough air leakage to allow motor cooling whilst achieving some vacuum.

<hr width=100% size=1>

 

[snowleopard]

Re: Vacuum cleaners...

true, though some have a separate fan to cool the motor and can be used in this way. the range of cylinder cleaners with the silly faces (charles, henry etc) have this.

with an ordinary cleaner you might need to bleed a little air in but the loss of vacuum won't be enough to prevent the bag doing its job.

<hr width=100% size=1>

 

[Guest]

A big thankyou to all of you for your interest and replies. It's great to have so much encouragement from fellow enthusiasts.

STH

<hr width=100% size=1>