Marine ply or exterior ply, pros and con's

alexrunic

alexrunic

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Marine ply or exterior ply, pros and con\'s

i am planing on doing a fair amount of wood work on my yacht this winter. most of it inside so was planning to use exterior grade ply can any one see a problem with this. also going to build an exterior locker lid / cover and was going to use exterior grade ply as well.

any thoughts on this would be appreciated.
 
Re: Marine ply or exterior ply, pros and con\'s

Some exterior ply is fine, some is not. If you get good ext ply, seal it with epoxy where it might get wet, it should last a long time. The locker lid I would consider covering with a layer of glass and epoxy, but the main thing is you want to keep the UV off it, including covering the epoxy with some decent varnish or paint.
Equally the other day I was using an offcut of what I'm sure was supposed to be ext ply, as a sanding block for wet sanding a rudder. I left it in a bucket of water overnight and it delaminated entirely.
 
Re: Marine ply or exterior ply, pros and con\'s

Basic rules of thumb for WPB: if its going to get damp or wet dont use it. This includes splash up from the bilges, drips from the windows etc.

If it has to take any structural load, or is likely to be under stress - dont use it

If you want a decent looking varnish finish to match existing - don't expect to use it (but you might be lucky and finmd a decent outer veneer.

If it is exposed to the weather - dont use it.
 
Re: Marine ply or exterior ply, pros and con\'s

I used exterior ply, to save money, for the decks and an engine housing. There were two or three main problems.

(a) the wood is much softer than marine ply, so when I screwed on fittings, they were not held nearly so securely into the exterior ply as previously they had been held into the old marine ply.

(b) the inner layers were not all complete, but significant areas of gaps were made up with a sort of paste filler or just left void.

(c) Despite sealing the faces and edges with epoxy, there was some water ingress causing some delamination of the outer skin in places.

In future, I would pay the extra and get marine ply as the amount of time and effort to do the job justifies the better quality material.
 
Re: Marine ply or exterior ply, pros and con\'s

Suggest you look on the Robbins Timber website as they do a range of plys for different applications. The key features of the differing grades revolve around the quality of the glue, the quality and number of the veneers used in the lamination, the number and size of voids permitted in the interior laminations (which you never see until it is too late) and finally the nature of the face veneers - decorative veneers such as teak ash or cherry up the price. So not a simple choice between "exterior" and "marine"

For interior work suggest you use a structural but not decorative unless you want to finish it bright as on a bulkhead. Anyway, look on the site and ask their advice.
 
Re: Marine ply or exterior ply, pros and con\'s

Trouble is until you talk to a timber merchants - you wont see the many different grades and qualities of each. Exterior grade ply can be truly excellent or downright awful. Marine Ply is not always prime, as that has it's seconds as well.

Timber merchants are the place to seek out. You get a guy in front of you who usually can advise better than most, can cut and select suitable ... price is usually good as well.

I am happy to use exterior grade for near all work - contrary to some posts here, but I use grades that have multi laminate construction - not the 3 or 4 ply grades that have filler block in and loads of voids. Using multi-layer laminate ... I mean the high numbered layer plys - you avoid voids, get higher strength per mm thickness and better bend-a-bility to form shapes.

Marine is not necessary if you go to the right people and talk to them !
 
Re: Marine ply or exterior ply, pros and con\'s

I went to my timber merchant today for some marine ply. He asked what it was for and then said that the "red and white" exterior ply is made with the same laminates and glue, but the facing veneer is not as good as marine. He said that as long as I sealed the edges properly, there should be no delamination.

Convinced me for my application and did himself out of £14! (£28 for sheet of 18mm)
 
Re: Marine ply or exterior ply, pros and con\'s

"red and white" exterior ply is made with the same laminates and glue,

Unlikely. WPB is almost always made with a thick core laminate or laminates of inferior grade of timber, usually softwood which will rot quickly. The facing veneer is usually very thin. True marine ply has laminates of equal thickness right through and of the same timber.

Exceptions in both types of course. Far eastern 'marine' ply even though it is stamped BS1088 is very often inferior core board, and just occsionally you will find a wpb which is nearly as good as marine.
 
Re: Marine ply or exterior ply, pros and con\'s

I was told once by a boatbuilder that he always personally tested a batch of ply before making any assumptions about its suitability. He advised cutting a piece about 3 " square and boiling it in a saucepan of water for a few hours. Then leave it on the hot boiler flue overnight.
Good marine ply will pass both tests without any sign of delaminating.
 
Re: Marine ply or exterior ply, pros and con\'s

[ QUOTE ]
I went to my timber merchant today for some marine ply. He asked what it was for and then said that the "red and white" exterior ply is made with the same laminates and glue, but the facing veneer is not as good as marine. He said that as long as I sealed the edges properly, there should be no delamination.

Convinced me for my application and did himself out of £14! (£28 for sheet of 18mm)

[/ QUOTE ]

Good for you ... I did similar in UK and out here - not regretted it at all.
 
Re: Marine ply or exterior ply, pros and con\'s

the boiling test as mention by a previous poster is the only one I would personally trust. Some marine ply is made of timber that is not durable but is allowed because it is lightwieght. eg gaboon/okume. There are many 'mahoganies' used, some better than others, in both WBP and marine ply. Good multi-laminate marine ply is never cheap but is worth buying unless weight is a consideration
 
Re: Marine ply or exterior ply, pros and con\'s

Unfortunately even Robbins marine ply can sometimes have small voids. Having had a £100 (delivered) sheet of 12mm Robbins Elite CNC routed into a number of components I was surprised to find holes in the internal veneers on some of the revealed edges.

The joinery that did the work were very complementary about the ease of producing good work from this ply and Robbins were very appologetic and said this ply should not have voids but in the event as the ply had been machined I had no option but to make good the edges with offcuts.

Nontheless, I would still use this for marine items that are exposed to the elements as I know of none better.
 
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