Painting a plywood hull

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2ply

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I've got a (16ft) plywood skinned runabout, and want to epoxy paint the hull. Old paint is falling off in sheets and taking the smooth surface of the ply with it - leaving a sort of roughened 'graining' behind. Should I try and sand this surface perfectly smooth before painting, or should I use some sort of epoxy 'filler coating' that can be sanded fair before topcoats? (I'm thinking a filler coat might stick better to the slightly grainy surface.) If so, is the filler coat something that can be bought off the shelf, or will it involve self-mixing microballoons(?) with something else... if so what? Thanks for any practical advice – I'm a novice with 2 part epoxy paints/fillers and such.
 
First thing is to determine if the ply is actually structurally sound. If the outer ply has come away it's a good bet that the glue has failed and the integrity of the ply is gone.
If the ply is OK it needs stripping and leaving to dry for a good 2-3 months. It must be absolutely dry to take the epoxy.
You need slow curing epoxy and to apply a straight resin/hardener coat to soak into the ply followed by a filler coat using microballoons and possibly wood dust to create a thicker (less runny) layer. If you apply wet on wet it'll create a proper chemical bond. This is why you need a slow curing resin.
Fairing epoxy is very hard work so get it as good as you can and wear breathing filters.
It may be easier and less effort to re-skin the boat with new ply and epoxy coat that.
 
Agree with Lakesailor mostly but more needs to be said.

Assuming this runabout will reach planning speeds when you hit a bit of chop some quite big shock loadings will occur, in this instance you if you are planning the epoxy route you need to be sure that any seams or joints are absolutely secure.

I would suggest stripping her back to a bare boat, turn her over and remove and keel or spray rails fitted, leave to dry for an extended period, when confident that she is dry refit (with epoxy) the bits removed and check that the joints where the plywood meets along chines or under keel are well made, withdraw some screws and see that condition they are in.

then proceed to epoxy coat.

similare practive could be carried out inside the hull.

Hope this helps.
 
Hi - and thanks for your response(s). It's not the outer ply that's gone, just a paper thin 'skin' off the surface where the old paint soaked into it.
 
Thanks for this BurgundyBen. She's been dry stored, so moisture isn't a problem - and I am following the bareboat route. The screws seem to be in good condition with the odd exception, but the seam down the main vee/along the keel has opened a little in places, I think mainly due to shrinkage or original filler falling out, because everything is screwed down nice and tight. I was going to flood the area with resin, fill any remaining cracks with epoxy filler and then apply a reinforcing fibreglass ribbon the full length of the boat before re-fitting the metal 'skid plate'. I've never attempted anything similar before, so pointers to any potential pitfalls would be appreciated.
 
Using epoxy or two pot on an old plywood boat is really a waste of money. It will not stay there however careful you are. Firsly you have already noticed movement between the frame and ply sheeting. This will move again almost immediately it gets wet, will crack the epoxy joint (yes, it will!) water will get underneath and all your hard work and expense will just fall off like the old paint has, only quicker!

IF - and its a big IF - the moisture content of the timber is no more than 15%, you might get away with using a sheathing system such as the West system, but unless you are absolutely certain the plywood really is dry, this too may well fall off again too!

But old Plywood and 2 pot paint just dont mix and part company very much more quickly than conventional paint systems.

Secondly, if the plywood has been exposed long enought to be weathered, then it is damaged. Again, you have already found seams that have opened up. The end grains of the veneers are exposed, will have taken up water, and will be beginning to detriorate.

Thirdly in all probability if the seams have opened then the glue joins to the frames are suspect, and need to be opened up for re-gluing. Pouring epoxy in may do a temporary repair.

The basic trouble is wood moves:it shrinks and expands with moisture, heat etc, while epoxies do not. something has to give - its always the wood, particualrly where plywood is involved.

A proper sheathing system realy is the only way forward - not the Polyester resin and Glass fibre one sees lamentably often on dinghies - which spells a rapid death from rot as water leaks in between the two materials and stay put, feeding the rot spores that are already there.

Much much better is to go for a traditional paint system and soft polysulphide fillers such as Sikaflex to fill the gaps, allowing the hull to continue to move as it wishes without opening up again. Buy a good quality primer, as without that whatever you put on will lift off again!
 
Thanks Oldharry – your advice is obviously best, as it's easily the longest!

Seriously, thanks for taking the time... I'm attracted to your method, not least because of its simplicity. The trouble is, I'm not sure I could get any useful amount of Sikaflex into the opened seams - they're not that wide and I'd end up with a messy external 'patching' effect – whereas epoxy would run in a treat. Although clearly that's not much use if it's going to crack anyway...

However, if I follow your route would it be worth flooding resin/hardener into the crack just to seal the plywood edges – which have indeed degraded slightly – prior to bedding the trim over the join with Sika? Or does resin simply not bond properly to plywood at all?
 
SWMBO says I always waffle on when it comes to boats....
/forums/images/graemlins/crazy.gif

Flooding with epoxy will certainly help seal things up, but how long it wll last depends largely on the state of the timber, and what glue was originally used to make the joins. Epoxy will not penetrate old glue, so may not penetrate rotten timber far enough back to reach sound wood again. If it does you have a 1st class joint. If it doesnt, it will all fall to pieces again!

A tip when using Polysulphides, put masking tape down each side of the joint. Once you have applied the Poly, run a (rubber gloved) wetted finger down the joint to smooth it and remove the excess. Then using a lint free cloth soaked in white spirit, smooth everything over to get a good finish. Remove the masking tape and you will have a near perfect seam. Dont let it set on your fingers or skin - it takes days to remove it!

<does resin simply not bond properly to plywood at all? > Yes it bonds very well,and is often used in making plywood. But the timber is very very dry to start with. Nearly always when an epoxy join does fail, it is the material it was attached to that has broken, not the epoxy itself. Thats why its not brilliant with old ply - the ply edges tend to detriorate, so if the epoxy only sticks to weakened timber that is where it will fail. Fine for gluing sound timbers, though most wood boat builders prefer Polyurethane glues nowadays for timber joints - again because Pu allows for more movement between the joined components than epoxy although it is not so strong. They will often choose epoxy when making a scarph for example where the adjoining timbers are unlikely to move to any degree and strength is more important.
 
[ QUOTE ]

Flooding with epoxy will certainly help seal things up, but how long it wll last depends largely on the state of the timber


[/ QUOTE ] Gets us back to my point that it may be as easy and definitely a better final job to re-skin the boat with new ply. I replaced some bottom on my dinghy and repaired the rest with epoxy to maintain originality.
I wish I'd reskinned it as the work involved in repairing was enormous.
 
You could well be right Lake Sailor - as I said, plywood that has gone grey is already damaged, and 2Ply has also found visible evidence of endgrain penetration - not good news.

If he really wants to 'restore' the hull rather than just keep it going for a few more years then replacing the sheeting is the best way forward. However decent marine ply is not cheap, and one has to ask if an elderly ply speedboat is worth the time and cost of complete reskinning.
 
Well, in spite of your heartless innuendo regarding the value of old plywood speedboats (how dare you, sir!) I intend to forge ahead regardless. I plan to use the old school method proposed by Oldharry, as I've all too frequently come unstuck when straying too far from the KISS principle. Oldharry's contribution may have come as a timely warning.

So I'm left with a single remaining dilemma – namely whether to go for a silver hull with red and white shark's teeth as originally planned, or the primrose yellow favoured by my better half. Do forum members have a view on this?
 
[ QUOTE ]

So I'm left with a single remaining dilemma – namely whether to go for a silver hull with red and white shark's teeth as originally planned, or the primrose yellow favoured by my better half. Do forum members have a view on this?

[/ QUOTE ]

Quite probably.... /forums/images/graemlins/shocked.gif
 
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