How dry should wood be for varnishing/painting?

[girlfriday]

Late last year I acquired a 12 foot wooden clinker-built Tideway sailing dinghy, which I am currently getting ready for sailing this year.

She had been out of the water for at least a year when I got her and consequently was pretty dried out. We made one attempt to sail her which had to be aborted as not surprisingly, she leaked like a basket! After that, on the advice of some people in the Tideway owners association, we tried running water through her for a few days and eventually she took up nicely and proved we have got a boat which might eventually float. This was in September and she has been in a dryish garage ever since.

Some of her varnish is in a pretty bad state and I am planning to strip back to bare wood in the worst places before painting below the waterline and revarnishing above.

My question is: would there be any benefit in repeating the soaking treatment before painting/revarnishing, or would this be a bad thing to do? The reason I am thinking it might be a good idea is that by allowing the wood to swell more before painting I would minimise the movement in the wood after painting and hence the possibility of the paint cracking. The reason I am worried it might be a bad idea is that it could trap moisture in the wood and lead to rot.

Basically, I suspect the wood is still in a drier state than it will be once she is being sailed regularly - it it bad to paint while in this state, or not?

Many thanks to anyone who can advise....!

 

[Mirelle]

It should be nice and dry!

Good question. The answer is that the type of paint used on conventional wooden boats (alkyd and chlorinated rubber based paints, and some one part polyurethanes) are able to move with the wood. Two part polyurethanes and epoxies cannot and will crack off as the wood swells.

The adhesion of the paint is better if the wood is nice and dry to begin with - indeed paint and varnish will not adhere at all to wet wood.

Some specialist paint manufacturers, such as Coelan, actually recommend using a moisture meter, but for normal paint just get the wood nice and dry!

Incidentally, about the worst single job in the whole of wooden boat maintenance is stripping the inside of a conventional clinker dinghy - once you have done that, nothing else is ever quite as bad!

 

[girlfriday]

Re: It should be nice and dry!

Many thanks, Mirelle - that's very helpful. It sounds like it's very important to make sure I am using suitable marine paint and not be tempted to skimp and go for whatever is cheapest in B&Q!
I take it, then, I should not be worried about the paint preventing the wood from taking up properly?

 

[Mirelle]

Re: It should be nice and dry!

No; you need not be. ordinary paints (unlike epoxies) are vapour permeable, so the wood behind them will adjust slowly to the ambient moisture level.

As to paint, the "marine" paints and varnishes - International, Blakes, Epifanes and such - are dearer but they do seem to hold up better.

There are differences in the chemical composition; the marine varnishes have a better ultraviolet protection ingredient, and so on.

 

[dickh]

Re: It should be nice and dry!

Just one tip - do not be tempted by the International Marine Varnish in B&Q(the one with a nice sailing boat on the tin) - reading the small print it's only suitable for use indoors! - NOT for external use - bit of a con IMHO.

 

[ccscott49]

Re: It should be nice and dry!

Just finished doing my clinker dinghy!! I hope I never have to do it again!!! Mind you he looks magnificent now!

 

[ianwright]

Pretty dry...

What Mirele said,,,,,,,,,,, plus a handy tip. Working outdoors and unsure if the day is warm/dry enough? Splash a little water on the old paint you wish to cover. Go away, drink tea. If when you return, say 10 mins later, the surface is dry then you may paint.
I wish I could afford a shed,,,,,,,,,,,,,,,,,,,

IanW

 

[seo]

Two part answer:
1) Pretty dry so that the varnish will stick,
but
2) pretty wet so that the wood isn't all shrunk back.

One approach is to soak the wood with numerous coats of oil. In theory the wood soaks up the oil, which fills the cells with semi-hardened oil that doesn't turn to vapor or ice with change of temperature. Also, since varnish is mainly oil, it sticks to it just fine.
Raw linseed oil works well, but will blacken in sunlight. But there isn't any sun in Britain, is there?
I suppose you could use Tung oil, but first you need to find a painter with a very oily tongue.
seo

 

[Mirelle]

good tip

I've an idea I was told that, thirty years ago, but had forgotten it - will now remember! /forums/images/graemlins/smile.gif

 

[trouville]

Re: good tip

dont forget to dilute at least 20% the linseed oil otherwise even English sun will do have an effect on the surface!

And if you varnish it the sun wont turn it black! But air and pollution from mobos lorries and buses will!