Why do We Varnish?

Well it’s not as silly a question as you might think at first, we do it of course because we are proud of the beautiful wood that our boat is made of and we varnish so that we can protect the wood, but the real question is, do we varnish to keep the water out…..or the sun out.
Of course the real answer is ….Both! We need to keep the wood waterproof or otherwise the boat would leak…and rot…and look horrible to boot, but in many ways the most important thing about our varnish is that it keeps the UV out and believe me, it is the UV that damages the wood…..and the varnish. That really is the nub of the thing.

OK, so we know that UV damages the wood and the varnish, but most harmful of all, it damages the varnish-to-wood joint. Varnish has UV blockers in it, so it protects the wood, and all the coats of varnish below it, but it can only provide so much protection and once it has been used up, the UV will get through and start to damage the layer of varnish below, until it gets to the varnish-to-wood joint. Once the UV works on that it starts to break down and the varnish starts to break away from the wood. Of course this then allows water to get in, which just speeds up the breakdown of the join….and the varnish….and you are shortly forced to remove all the varnish back down to the bare wood to have any chance of getting a good result

This is why old varnish can be seen to just peel of in sheets, although the varnish itself isn’t too bad.

How do we prevent this……easy…we never let the varnish fully loose its UV protection. So the answer is simple, you need to sand back a coat of two and replace them with new varnish…..every year? In my humble opinion if the boat spends lots of time in the sun, I would do it twice, a couple of coats in the spring and an extra one in the autumn, which will help seal any cracks and things before the weather deteriorates. If you use a polyurethane, the argument is that you can leave it longer between coats….and maybe you can….but whatever the varnish, you are going to have to re-coat sometime and traditional varnish is much, much easier to remove and repair between times. It is worth remembering that you can always use a traditional varnish on top of a modern polyurethane but not the other way around, so I often I like to make a good base of hard polyurethane varnish and then top it off with traditional varnish.

so…..the important thing is to always keep your varnish topped up and regularly sand down and apply a new coat or two of varnish to keep your UV

protection up….If you don’t do this, then the varnish will keep out the water for a while longer, but sooner or later it will start to go wrong.

But I can hear grumbles of dissention in the ranks… you say……if I put a couple of coats of varnish on every year, then in a few years it will have 3 tons of varnish. Well I know it sounds like it…..but it never seems to work like that, as the UV breaks down the varnish, it oxidises and thins and it is easy enough to sand off a couple of coats of varnish before you put the new varnish on. You can go on building up varnish, and as long as you sand between, it just gets flatter, and better looking every year. There will come a time, when you need to take it all off and start again, at which time you will be very thankful if you decided to use traditional rather than polyurethane varnish.

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