hull finishing

Hello all first time here so go easy, I'm building a laser sized dinghy from plywood I'm trying to work out what to use to waterproof the Hull, the dinghy will be out of the water most of the time. An ideal coating would ideally be a ronseal esque varnish or a waterproof paint product as I have no fibreglassing experience, also one final request please don't tell me I need epoxy as I simply can't afford it. So any ideas? Cheers, Jack

 

All the other coatings aren't water proof Jack. Epoxy isn't truly waterproof, but it is waterproof enough to prevent moisture gain in encapsulate wood. Nothing else does this, as they all let in enough moisture vapor to change the moisture content of the wood (well, off the usual choices).

Varnish (alkyd) is well down on the list in terms of waterproofing wood, single part polyurethane is better, but not as good as shellac. Two part polyurethane is better still, though still not waterproof. Last on the list other then epoxy is polyester and vinylester resins, which also aren't waterproof, though vinylester is close. Traditional oil finishes (Dutch oil, etc.) are at the very bottom of the list, below flat acrylic (latex) paint.

What build method have you employed on your dinghy, as this can really affect the choices you have?

 

Thanks PAR, I believe that's one of your better posts!

 

PAR thanks for your input, ive been browsing the forum a little and you seem to be top boy on here, so your advice is much appreciated. After a little research i think in the end it may be worth the extra expense of epoxy and glassing the hull. if anyone could recommend a good place in the UK to buy epoxy that would be much appreciated. Cheers Jack

 

You don't have to use glass when you epoxy...you can simply coat with the resin (2 min 3 better)then paint over that. You will have to carefully monitor the bottom for scrapes and gouges that go through the coatings and repair them promptly...but you have to do that with glassed ply too.

 

Thanks for the input Lewis, can epoxy be applied like a normal paint? Or does it require special treatment?

 

The beauty of using a thin layer of glass fabric is that when applied and finished with the weave filled , it automatically develops a consistant film thickness of epoxy over the complete structure . Epoxy achieves its best waterproofing ability when it is applied to the correct film thickness.

 

If you are using BS1088 ply, worth spending money on Epoxy resin coating. If you are applying epoxy in humid condition, cover with plastic film straight after application or epoxy will under cure as it reacts with moisture. A cheap option is to use epoxy based primer like Sea Jet 117, this high build primer act as surfacer so you can apply top coat after sanding.

 

Unless you are applying epoxy in the rain, then humidity isn't going to "mess up" a bond or coating. Though it can affect cure rates and cause a bigger amine bloom (amines are water soluble). Covering with plastic sheeting is a good idea, it will protect the coating from most thing, like bugs, dust, ambient conditions other then temperature, etc.

There isn't a "correct film thickness" to be waterproof, but there is a minimum that you can say will not be, which is about 10 mils. Any amount over 10 mils thick can be considered water proof. This is precisely why I recommend at least 3 full coats of goo on a surface. You might get by with 2, but you're risking the potential of "thin" areas, where the wood sucked up more resin that other areas, coming back and haunting you later.