Voids in plywood question

[Pipefitter]

I keep hearing about voids in plywood. I can almost understand why this might make it bend funny but it seems it would have to be quite large.As far as condensation forming within the plys,how is this possible?I mean,why would this occur moreso in a boat than say hanging on the side of a building or on the floor of a house? Are these voids not encased in the gluelines of the plywood? If exterior grade plywood has waterproof glue,wouldn't this take a long time? I was always told upon questioning a builder's using marine grade fir exterior for a workboat, that I would spend more time repairing damage from scratches with rocks or oysters or getting sanded from beach landings then I would fixing a rotten void.He claimed if I found rotten wood in a boat soon enough,it was no more than cutting in a cold patch(neat little trick he showed me using multiple sizes of hole saws where the wooden puck has a rabbet and the hole cut in the boat has a rabbet as well, creating a male and female part with a large glue area)that is glued in place and glassed over.

Maybe this void issue means more on a boat that lives in the water? How many of you have seen rotten voids in plywood?

[Tom Lathrop]

Pipeman,

Rot requires moisture and oxygen.  Take either of these away and there will be no rot.  That is why the term "dry rot" is an oxymoron and can not occur.

All wood has voids within the cellular structure.  Red oak has more than most and rots quicker than white oak which has blocking material in the veins.  Balsa is full of air so it rots very quickly.  However balsa core that is full of moisture gained by osmosis through a fiberglass skin but, is otherwise sealed from oxygen, is just wet and heavy, not rotted.

So an enclosed void by itself is only a mechanical problem and not one of rot.  Lots of boats have been built with Nomex, a honeycomb material that encloses mostly air between wood skins.  Not saying that there have not been rot problems there though.  A linear void that runs from edge to edge in plywood is the worse case since a breach anywhere along the void can let in both air and water.

I don't like voids and try to a-void them but they are probably not the buggaboo that many assume,

[umop_apisdn]

Ok, I don't know much about this, but that isn't going to stop me from voicing my opinion on the subject.

I've always wondered about this too, since lots of boats use a balsa core, which is mostly air anyway.  I came to the conclusion that the problems with voids must be primarily structural or mechanical.  Small ones probably don't matter much.  

That said, were I to build a boat from scratch (which I have NOT done, so take my opinion with a grain of salt), I'd still definately go with marine ply. You get more plys, stronger glue, it will bend more evenly, and have a higher strength-to-weight ratio.  The fact that is has no voids is probably a good thing, but not the main reason for choosing it, IMHO.    

If you are going to invest 1000 hours of your time into something, you want it to be made out of the finest materials available.  It would really suck to have a long, straight void right in the middle of a bend, and have a "crease" appear in the hull of your boat over the top of it after a year or two....

Noel

[Arnie Eaton]

There are more grades of marine grade plywood then AA. I buy AB and pay a lot less then AA grade. The "B" side goes to the inside of the boat in most cases but can also be on the outside if you glass. There are NO voids in marine grade plywood even with one side graded "B". B grade can have plugs within the surface ply. but finishes the same as "A" side.

[Barry Pyeatt]

A recomendation if you are purchasing AB Marine and plan on glassing over one side of it.  Put the "B" side out to be glassed.  Keep the "A" side to be finished as you choose.  

I see builders all the time trying to get a really nice finish on the lesser graded side when they glass over the good side.  Glass and resin don't care if it is an "A" surface or a "B" surface.  No, there isn't much difference between the sides perse', but again, down the road after some time out in the weather, it helps to have the better side inside the boat with a good face to paint rather than dealing with a plugged surface veneer.  

Just a little tip from someone that hates having to repaint interiors just because the plugs  outline shows thru the paint.  

[Pipefitter]

I would agree to get the best you can.Atleast below waterline. I hear of alot of people using MDO for it's fine surface for finishing. That paper or glass/epoxy has to be a much better veneer than even the most expensive wood. Either way it is at the least that you're gonna pay now or pay later one way or the other. I had not considered how much my time was worth when I was having to spend whole weekends sanding the lesser quality materials.