View Full Version : 'Stabilizing' wood for varnish
I was wondering if anyone had any advice for ‘stabilizing’ wood that will be varnished over. ie: minimizing shifting and resettling (from climate change or whatnot) where two pieces of wood meet so that the varnish doesn’t crack and expose the seam. I refinished part of the brightwork last summer and one of the wood interfaces cracked wide open and is now leaking a bit. I want to not make this ‘mistake’ again.
Is there anything I can do? I was thinking about giving it a good drink of oil (linseed, tung) before sealing and varnishing… any thoughts?
05-21-2006, 06:00 PM
What a question to ask? ... you too hey? My answer is as a painter of old boats ... this problem is what painting old boats is really about.
I have that trouble from time with dry old timber that hasn't seen water and then the first hint of water ... they go off in the end grain, no matter how well I've sealed an area generally ... if I can't seal the end grain to a saturation level and I have to expect it. Here is the perfect case for using CPES in an attempt to penetrate some fine old joins.
In almost all of the cases that I've encountered (and exactly what I'm working on today) it was because the timber that was painted was not fixed adequately to the supports and maybe wouldn't have been even noticed or even moved as much if they had have been newly assembled, well glued, or left unpainted but the broken edges get sharp when the varnish cracks and also feel dangerous.
In my case, I don't know that it will happen often because the boats aren't mine that I paint, so I have no history with them. I've written here on a few occasions, that I like to keep a boat that has been painted, for a while after painting to let things cure properly or revisit the boat a few times after painting because of this very same thing happening if they are having their first swim for a while.
Mate you just have to deal with each problem as an individual problem when one arises, I do otherwise I'd be pulling boats apart continually. I just expect these events because wood is a sponge, no matter how well you seal it and top coat it. I try to stabilize the timber by giving it time to settle down and sealing joins well and allowing the sealants to shrink before painting.
The more you can rehydrate the timber with oil the greater your chance of keeping the water out, but this in itself has problems. When painting with oil based enamels or varnish (because the other paint bases are out of the question over oil, I haven't seen alkyd resin marine paints here in Oz). When you paint with oil based materials the underlying layers draw oil from upper layers within the paint and varnish. When the top layers become dry or depleted of oil they crack. The ideal is to have the most oily (the oiliest) layers as the top coats. This is known as painting 'thick over thin' or 'fat over lean'. You add a bit more oil to each layer or progressively reduce the solvents in each upper layer, ... as you paint your way to the top. So your most oil rich coats are needed on top. You want the most flexible layers on top.
The other way that many swear by is to stabilize the wood with tung oil or CPES right from day one if you are building. I'm not sure how many boat boys do this, using tung oil ... but the top end timber workers do, those who make the finest of wooden products. I use CPES.
05-21-2006, 06:45 PM
I bet very few use tongue oil as it is very hard on the donor. Tung oil, now there's a product!
Warren, apologies, I'm not normally in the business of punning with spelling errors; this was to good to pass up.
I'm not sure how many boat boys do this, using tongue oil ... but the top end timber workers do, those who make the finest of wooden products. I use CPES.
05-21-2006, 06:50 PM
Well, what we are discussing here is an on going problem that begins with a basic failure to begin painting or varnishing over inadequately prepared surfaces. If a builder knows the way wood comes and goes with changes in humidity and temperature, he has already done proper prep work for the painter in that seams at butt joints and scarfs have been properly sealed, luted, caulked and payed. Then it is the job of the finisher to lay on the various primers and paints or sealers, stains and varnish. It should be noted that at no time should seam compound be painted over or varnished if they are not to end up shedding or cracking their finishes. A good painter, who is worth his salt will cut in the seams and not lay paint or varnish over them. There is no way in hell that a painted or varnished seam on covering boards will not end up cracking over time. Topside seams will eventually crack no matter how much care is applied to a planked hull. That is unless the hull is splined and even then some cracking can be expected. One trick to save varnish that has begun to seperate at a seam is to apply varnish with a hypodermic needle under the blister or separation.
05-21-2006, 06:57 PM
Thanks Lew, fixed. I now have it licked.
05-22-2006, 12:44 PM
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