DIY bench seat plywood

[Blankton]

Hi all. Another winter project I'd like to do is replace a bench seat on my sib. I made the current seat myself from a piece of 22mm ply and a seat kit (glue on patches and the plastic angle things).

It was just standard ply, not the best quality, but I filled any gaps at the saw cut edges and painted it with three coats of "marine grade" paint.

I don't really sit on the seat (I sit on the tube) but I have my fishfinder, GPS and battery fitted to it.

Anyway, after a year, it is completely trashed. All of the ply has delaminated and I can now bend the seat like a banana.

I need to re make it, but hopefully I can make it last a bit longer this time.

Would marine ply have made much difference? Or is there something else I should have done, such as seal it with epoxy before painting?

[Steve509926]

Marine ply will make a big difference. However, unless you keep your sib inflated with the seat in place, outside and uncovered, you don't need to go to the expense of "true" marine grade ply. Good quality "external" grade hardwood ply is more than good enough but as with all wood, it has to be treated correctly. The following is the way I would do it, others may suggest doing it differently.
Firstly plane and sand the edges making sure all edges have a decent radius. At this stage don't fill any gaps at the edges with filler. Seal and prime your bench with Zinsser B.I.N shellac sealer primer making sure all edge gaps are well covered, do at least 2 coats on the edges. Use cheap paint brushes and throw them away and wear gloves, it's a pain to clean off. Now you can fill any gaps at the edges with Plastic Padding car body filler. After a light sand down do another coat of Zinsser. Now it's ready for an undercoat and a couple of top coats of good quality oil based external/marine paint.
If you are going to drill any holes in the bench do them before the first coat of zinsser.

[beamishken]

Originally Posted by Blankton

Hi all. Another winter project I'd like to do is replace a bench seat on my sib. I made the current seat myself from a piece of 22mm ply and a seat kit (glue on patches and the plastic angle things).

It was just standard ply, not the best quality, but I filled any gaps at the saw cut edges and painted it with three coats of "marine grade" paint.

I don't really sit on the seat (I sit on the tube) but I have my fishfinder, GPS and battery fitted to it.

Anyway, after a year, it is completely trashed. All of the ply has delaminated and I can now bend the seat like a banana.

I need to re make it, but hopefully I can make it last a bit longer this time.

Would marine ply have made much difference? Or is there something else I should have done, such as seal it with epoxy before painting?

Must have been something up with the ply you used as even standard exterior ply should last better than that. My pal builds wooden rowing boats for his hire fleet that sit in the river Wear in Durham all summer. He uses marine ply for the clinker planks of the hull but for the internal seats & floor boards he uses standard wbp exterior ply. The boats on average last him 25 years & its usually the underwater planks that finish them off.
Your seat if finished with a decent paint or varnish should last a good few years made from standard Wbp exterior ply

[69cmw]

Why not use HDPE instead?

[Steve509926]

Originally Posted by 69cmw

Why not use HDPE instead?

Where's the fun in that

[Steve509926]

Originally Posted by beamishken

Must have been something up with the ply you used as even standard exterior ply should last better than that.

It may not have been exterior grade ply, and if the edges aren't sealed correctly ply breaks down very quickly.

[Blankton]

Originally Posted by beamishken

Must have been something up with the ply you used as even standard exterior ply should last better than that. My pal builds wooden rowing boats for his hire fleet that sit in the river Wear in Durham all summer. He uses marine ply for the clinker planks of the hull but for the internal seats & floor boards he uses standard wbp exterior ply. The boats on average last him 25 years & its usually the underwater planks that finish them off.
Your seat if finished with a decent paint or varnish should last a good few years made from standard Wbp exterior ply

This was my thoughts, that id get a few summers out of it in that application, even if it wasn't the "proper" stuff.

I think sealing of the cut edges is something I will need to concentrate on if using ply again.

I'm not against using some kind of plastic, its just a matter of sourcing something for a reasonable price. Something like chopping board material would be good, but the price seems to get very high when looking for material in a suitable size.

[treerat]

Are you just talking the seat IE a plank or the complete box with seat on top?

If the latter composite decking seems the way forward to me, and in a small sib a single width will give you enough width - and you'll only need a small offcut.

If the whole box - I have a fibreglass one that'll do what you need, just needs the seat bit adding.

[Blankton]

Originally Posted by treerat

Are you just talking the seat IE a plank or the complete box with seat on top?

If the latter composite decking seems the way forward to me, and in a small sib a single width will give you enough width - and you'll only need a small offcut.

If the whole box - I have a fibreglass one that'll do what you need, just needs the seat bit adding.

Just a plank.

I have thought about making a box seat / coolbox combo, but that's another project in the future!

[Steve509926]

Originally Posted by Blankton

This was my thoughts, that id get a few summers out of it in that application, even if it wasn't the "proper" stuff.

I think sealing of the cut edges is something I will need to concentrate on if using ply again.

I'm not against using some kind of plastic, its just a matter of sourcing something for a reasonable price. Something like chopping board material would be good, but the price seems to get very high when looking for material in a suitable size.

1000x250x20 HDPE - £52 delivered

1200X250X22 WBP External ply, Zinsser, paint etc - approx £35

It's not that much more to pay for something that's going to last.

[smallribber]

Originally Posted by Steve509926

1000x250x20 HDPE - £52 delivered

1200X250X22 WBP External ply, Zinsser, paint etc - approx £35

It's not that much more to pay for something that's going to last.

Are you saying wood dont last...... There are a lot of ship and boat builders who might argue with you there

[Steve509926]

Originally Posted by smallribber

Are you saying wood dont last...... There are a lot of ship and boat builders who might argue with you there

It's a bit early in the morning to be obnoxious!

[Steve509926]

Originally Posted by smallribber

Are you saying wood dont last...... There are a lot of ship and boat builders who might argue with you there

If you would take time to read the post right the way through you would see that I have suggested ways how to prolong the life of ply.
Do not try to lecture me on a subject which I do, for once, have a modicum of knowledge.

[smallribber]

Originally Posted by Steve509926

If you would take time to read the post right the way through you would see that I have suggested ways how to prolong the life of ply.
Do not try to lecture me on a subject which I do, for once, have a modicum of knowledge.

I am so sorry I dared question your knowledge my liege....... how very dare I

[Steve509926]

[jeffstevens763@g]

You could use exteria ply cover it in one layer off fibreglass chopped strand mat say 300 g then when it's cured rub down and flow coat it use a roller to get a textured finish to stop slipping

[smallribber]

There is one heck of a lot of overthinking going on here in my view

[jeffstevens763@g]

worth a look see if you can make it work





https://www.marinescene.co.uk/produc...BoCausQAvD_BwE

[Blankton]

A bit of an update on this thread. I decided to try HDPE as has been suggested, as it should be completely rot resistant. I was a bit worried about strength, so I went up to 30mm thick, as it was only a couple of pounds more - about £60 for a large enough piece.

I got a shock when the parcel arrived, I could barely lift it!

I have just cut it down to the same size as the old seat and its still heavy.

Wood = 2.5kg, HDPE = 5.5Kg.





I have been able to cut it with a circular saw and chop saw. It doesn't really like being sanded, I think in hindsight the router would have been better to break the sharp edges.

[jeffstevens763@g]

I use a de-burring tool on the edges and run a heat gun over the surface to take out scratches for info