Monday, July 22, 2013

That annual task

It's that time of year again - when the days of warmth and sun stretch out forever in front of us.  Days when you can absolutely depend on not having rain for two or three days in a row.  So that means lots of wonderful sailing, right?

Well, no.  It means that you can depend on not getting rained out in the annual varnish task.  It goes something like this:

  1. Mask everything with 3M 2080 tape - the kind that has an adhesive that tolerates sunshine and moisture (rain, or dew).  The regular 2090 tape becomes impossible to remove after a couple of days of sunshine... 

    Make sure that your masking does not actually cover the joint between the wood and the substrate.  Hold the masking 0.5 mm back, thus allowing the varnish to provide a seal between the wood and the substrate.  There is every reason to expect that the back side of the wood was never varnished.  If moisture gets behind the wood, it will soak into the wood and will lift the varnish.

  2. Sand everything.  I use 150 grit open coat aluminum oxide sandpaper from Norton.  I've tried a lot of others, but this is the best stuff I've discovered.  It doesn't load up with the dust. 

    I do all my sanding by hand, using hand-sized quarter sheets cut from the standard 9x11 sheets.  Each of these hand sheets gets folded in half each way, and then one of the creases is torn to the center of the sheet.  This allows me to fold the paper into a 2.75x2.25 inch pack which is perfect for hand work.  By making the tear, the pack can be folded in such a way that the grit sides of the unexposed paper do not touch each other, which would dull the grit.

    The objective of the sanding is two-fold - to provide a good 'tooth' on the surface so that subsequent coats of varnish can mechanically bond to the surface, and to remove the surface imperfections from last year's coats.  Yeah, there always are some.

  3. Wait for the perfect weather.  For me, this means a calm, cool morning.  If it is windy, I find that the varnish is drying too quickly and I get visible joints between the sections because I cannot keep a wet edge.  If the sun has been shining on the surface for long, it also causes the varnish to dry too quickly (see  item 2).

  4. Apply the first coat of varnish.  Pay attention to the consistency of the varnish.  You may have to add some thinner to allow you to keep a wet edge.  Also, if the varnish is too thick, it will go on too thick, and will result in wrinkly areas where the surface cured before the bulk underneath.  These of course can be sanded out next year... (see item 2)

  5. Wait 24  hours.  This allows the varnish to cure sufficiently that the brush won't drag when applying the next coat.  But not so much that the next coat won't chemically bond.  If you wait more than 24 hours, then you will need to sand again to promote a solid bond.  But this time you should use 220 grit.  The sanding scratches from 150 grit will disappear after two coats; the sanding scratches from 220 disappear after a single coat, presuming that it hasn't been thinned too much.  Having dependable weather is sooo important because it allows you to avoid having to sand between coats. 

    Be very careful to not leave holidays or gaps between sections (see item 2).  This is much more difficult with the second coat than the first, because the matte finish from sanding gives a great background for the first coat.  It helps to pick out a series of landmarks, "OK, this section stops at the bung".
In the past, I applied 3 coats each year, the reasoning being that the sun gets one coat, I sand off one coat, and so I would be adding a net one coat each year.  This is good reasoning after recently taking things back to wood, when the varnish film is relatively thin.  But after years and years of this, the film gets so thick that it cannot move with, stretch and shrink with, the underlying wood.  And so it becomes prone to becoming detached.

I have also concluded that the hardness and stiffness of a urethane varnish film causes similar problems of not being able to move with the wood, thus resulting in premature failure and detachment of the film.  I am no longer using urethane exterior varnish.

It's a shame that it takes so many years to build up an experience base that allows you to get good brightwork.  I am still learning.  I will probably take everything back to wood next year so that I can erase all the accumulated mistakes made in the past 15 years and start over.  If, that is, I can find the ambition next year...


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