varnishing mahogony

coastwise

coastwise

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Hartlepool....... NE
www.coastwise.org.uk
Would appreciate a bit of advice -

I know it is the wrong time of year for the job, but needs must.
I am stripping down the old varnish on the mahogony hatching timbers that run full length a foot above the waterline while my GRP old Nelson 40 is briefly out of the water for shafts.

There is considerably blacking at the damp parts around fastenings. As/if the timber dries out, will the blackening fade or will it need bleaching - I seam to remember using oxalic acid on oak ?.
What would you consider to be minimum temperature for using Goldspar polyurathene? The boat is not under cover and only has the sun on one side for 4hrs per day at present.

Would you consider a better alternative would be to strip down, apply a clear primer ( UCP) and varnish her when a bit warmer after launching? She needs to be back in the water by end February.

Any suggestions gratefully received.
 
don't know if this helps much, but i spent the morning flatting the cabin sides and putting another coat of varnish on again.

had a bit of sun to help, but did have problems with the runs.... in the varnish!

i think adding a little white spirit might have helped, as i haven't had the same problem inside.

have a look at the pics on my last post, and judge for yourself.

as for the dark bits, mine went when they dried out.... and she was a bit wet.... after she sank!

good luck.

steve.
 
Apologies because my boat is not a classic, but I have a blackened area on mahogany faced ply in one cabin. That area has clearly been wet in the past, but is now completely dry, and still black. I too am wondering if oxalic acid will work.
 
Oxalic Acid may take some of the blackness out. But it's far more effective on softwoods. It's worth a try, it's not going to get any blacker. Just don't be suprised when it doesn't completely remove it.
 
If your black bits won't sand out, cut them out and scaph a new bit in. You can't make an old boat look like a new boat but you can make it look like a well maintained old boat - decent scaphs look OK. I wouldn't bother with oxalic acid on hardwoods.

ALWAYS stain mahogany. It's the only way you'll get new wood to (nearly) match and the boat will look good for longer. Stain prevents or slows the sun reducing the wood to a flat blond shade. I use Interstain.

Don't thin your varnish - heat it up. This is particularly true if you are using Epifanes - which seems to be highly temperature sensitive. There are those about who say you have to heat it to get a good finish. To provide a perspective, the boys in the West Indies put it in the fridge. By way of unsolicited testmony - I think it's the best varnish in the world.

Old Frank
 
As the blasckening is restricted to spots around metallic fastenings, check for soundness of the wood where it is blackened. There may be some electrolitic decay going on. Is anything unusual happening with your anodes? On cold days it is a common practice to stand your tin of varnish or paint in a bucket of hot water to get it to flow better. Obviously, the water has to be replaced fairly frequently to keep it warm.
Peter.
 
Thanks for all your input.
She is a 1971 hull - ex inshore fishery protection, so she has had a few knocks in her time whilst boarding. The timber is pretty old. There is always a problem keeping water out from behind the hatching with the flexing of the hull.
I believe the damp black marks around the fastenings is from water ingress from both the front and from water trapped behind the timber rather than electrolosis.
As she is a working boat & will no doubt have many more knocks, I think I will try a bit of bleach to reduce the blackening, and polyurethane varnish when I can get her a bit dryer.
Thanks for all your advice on the different aspects - much appreciated.
 
I meant to add a warning about UCP to my earlier posting. It's great stuff for a yard that wants to get four coats on in a warm day to clear the shed - but that's about the limit of its value. UCP is very hard and extremely difficult to flatten to a decent finish. It's a pig to strip off again and varnish doesn't stick to it well - where you have abrasions (such as dog claws on the cockpit seats!) the varnish will come away from the UCP below.

If you want to use abrasive paper rather than wet & dry, you'll have to leave the varnish for a while to harden. Even in the summer, dry abrasive fills up if you try and rub down apparently dry varnish within 24 hours. Oh, and always use a block for rubbing down.

I'm not familiar with the polyurethane you mention - but I wouldn't use two pack on solid wood - the water can't get out again....... and you get black patches! Fine for ply but scratches easily and is also a piggy to remove. UCP will have a similar effect.

Old Frank
 
Re: water based varnish

does anyone know much about these water based marine varnishes...two coats in one day would be nice. Can they be applied over sound oil based varnish?
 
Time you met Coelan the Barbarian

Bring plenty of £££.... Und You Must Follow Ze Instructions!!

O/wise, Old Frank, him very right...
 
Re: water based varnish

Yep, I have used them for about four years now, easy to use, can give you a high gloss finish, two - three coats in one day and can even be used on the floor. Approx five coats and it will look as good as varnish!

However, they are not as forgiving as varnish to scrapes and knocks but are very easily repaired, even in poor weather conditions.

Have a look here
www.woodsealer.co.uk/marineclearseale.html

Tom
 
Re: water based varnish

I strongly suggest that you might like to avoid using International Paints yacht varnish.



Why?



Well if you go to their site ;

http://www.international-paints.co.uk/details.php?productid=93&search=1



and scroll down all the way to the bottom you will find :

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<span style="color:red">Hints</span>

Not suitable for floors or <span style="color:blue">marine use</span>.

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Ok, I've recently restored the whole mahogany cabin on my 1964 sailboat, so I have some imput. Oxalic acid WILL work wonders, but you have to do it right. First, you have to buy the powdered acid, not the pre-mixed liquid versions. This stuff is gnarly on the skin, so wear gloves! You'll also need distilled water both to mix with the O.A. and to rinse the O.A. off when it's finished. This is VERY important. The tannins in mahogany react with the minerals in tap water, creating more black areas. Put about 1/4 cup of dry O.A. in a ceramic cup or bowl. I filled a large spray bottle with the distilled water and sprayed water into the bowl just enough to create a thick, pasty mixture. Apply with coarse brush, using the tips to cover grain. Now, the key is to keep the O.A. wet for as long as possible. I re-moistened with the spray bottle several times throughout the day. Leave on for 1-2 days. When you go to remove the acid, remember to use distilled water. Rinse very well. My mahogany was in terrible condition, so I repeated this process about 3 times to get desired results. Good luck!
 
Arima - thanks for that.
I tried some oxalic acid yesterday. It was crystals for bleaching. I mixed with ordinary tap water & a drop of washing up liquid as per instructions. Painted it on, then rubbed off with a damp cloth a couple of hours later, then warmed the area with a hot air blower to dry off the water .
It worked well - two sessions about cleared it , then the snow set in.!!!
Thanks for your input. I will try distilled water on the next bit.
 
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