Wet Ply : How do you know if it's too far gone?

W

wipe_out

Well-Known Member
Joined
23 Feb 2013
Messages
1,399
Location
Bournemouth
Visit site
So clearing out our "new" boat this weekend we cleared out the V-Berth in the front and found that the previous owner had lined under the cushions with plastic sheeting to keep the cushions dry rather than find the source of an obvious water leak.. So this means the plywood that makes on the construction on one side of the v-berth is wet and judging by the black gunk on the underside of the wet area it has been damp/wet for a while..

How do you establish if the plywood has lost it's structural integrity or if its fine to just clean it up and dry it out?

The idea of having to rebuild the whole forward interior is not very appealing especially because we would likely have to remove half the interior to get to everything..
 
Thanks for the suggestions.. I have a dehumidifier running already.. Planning on putting a heater in there whenever I am there to try and speed up the drying process..
 
Failed ply is easy to spot. The sheets can ripple, or have bulges / blisters that are quite large, also the individual plies can separate at the edges. If they have separated at the edge, then use your finger or a small blade to investigate by pealing back. If the ply pulls away easily then damp / wet damage has destroyed the glue layer, if still firm it is just localised edge damage. The water may be from condensation between the warm mattress and the cold underside, that is very common, rather than a leak. The cure for that is either a grill, a mesh device (both under the mattress or holes drilled in the base panels. All work well at stopping condensation. If you have leaks, cure them, obviously.

If you do have damaged ply, it may not be a disaster, as bad sections can be cut out and new ply glued in, rather than whole scale dismantling and rebuilding. The repaired section can be hidden with paint or a thin sheet of veneer refacing the whole area.
 
wipe_out said:
Thanks for the suggestions.. I have a dehumidifier running already.. Planning on putting a heater in there whenever I am there to try and speed up the drying process..
Click to expand...

Also treat the whole area, and any new wood, with a wood preservative . Boracol perhaps.??? (10RH if you can get it otherwise 5RH.)
 
If you do have to replace a patch, cut your patch to adequately cover the damaged area, then use it to mark out the area to be finally cut out. screw and glue some pieces of suitable material, (ply or timber), around the underside of the area to be repaired, overlapping the hole, then fit the patch. (Apologies if teaching you to suck eggs :) )
 
To get a rough idea, use a light mallet or screwdriver handle to tap along the suspect area of ply to 'sound' the wood. Test on a known solid piece to get how good ply sounds. Soft or delaminated ply gives a dead 'cardboard' sound compared to solid wood which 'rings'. This gives an idea of the extent of any damage, but can miss recently attacked areas that havent fully softened yet.

Having established where any suspect areas are, use a spike or small bradawl. A small electricians screwdriver does just as well if the blade is worked with the grain, not across it. Try to push the spike into the wood. Dont be too violent just use reasonable hand pressure. If the wood is sound the spike will not penetrate more than around .25 - .3mm. If the plywood has softened, the spike will sink in quite easily.

The general wisdom is to go at least 30 -50cms further out from any ply identified as damaged in this way to get back to sound timber. Ply rots from the inside out, so it will have worked its way in along the inside laminations. As above there is no need to remove the entire sheet if damage is localised. Thats the beauty of working in ply, its straightforward to just replace a damaged area.

Various techniques available: A basic butt joint can be made by fastening backing pieces on the blind side around the opening, forming a ledge around 50mm wide on to which the new timber can be fastened. If there is likely to be much loading, such as on a bunk top, increase the overlap to 75mm. I.e the new supporting strip needs to be 100 -150mm wide, half on the old and half supporting the new. This can be glued or glued and screwed, but without screwing make sure the new ply is tight against the old using G clamps etc.

Another way which requires more skill is to scarph the new ply on to the old by chamfering the edges to at least x3 the thickness of the timber. If you can manage x4 thickness that is stronger This requires some skill to get a good join though, but the end result if it is well done is known as an 'invisible repair'. Only truly invisible of course if you can match texture and grain of the original!

Best glue for ply is Balcotan, rather than epoxy, as it is moisture cured. You are unlikely to be able to get the moisture content of the original down below the max 15% moisture content required for a good epoxy joint. A properly made Balcotan joint in ply is significantly stronger than the original, and the timber will break before the joint.

But before you start find out and cure the leak, otherwise you will be doing it all again in a few years time!

30+ years experience of patching up old plywood boats.... so got the tee shirt on ply repairs!
 
Thanks for all the advise.. Hoping that with a bit of time running a heater in the forward berth we can get it dry enough to try and determine the extent of the water damage.. By the sound of it just replacing the sections that are soft (if any) will be far easier than my original fear that we would have to replace the entire section.. The issue that remains is if the section to be replaced is glued/epoxied/glassed to the hull in which case it will be a bit more complicated..
 
You must log in or register to reply here.

Share this page

Top