Old school way


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Posted by Sherman in Idaho [108.162.245.23] on Tuesday, July 12, 2016 at 19:06:26 :

In Reply to: Re: Help with wood floor in Carryall posted by WarEagle [108.162.237.58] on Tuesday, July 12, 2016 at 16:57:20 :

If nothing else, a good starting point for any kind of priming of wood is to just take some of the intended finish coat and dilute it down with more solvent so it penetrates better. Basically, with paint you have solvent, vehicle, and pigment. With conventional oil paint, the vehicle is usually boiled linseed oil and/or soya alkyd resin. Both are plant oils that harden slowly by oxidizing when exposed to air. The solvent is usually a light petroleum distillate. In the old days it would have been gum turpentine. The purpose of the solvent is to make the vehicle spreadable and get it down into the wood. Too much solvent on a surface coat makes it thin and weak. But more solvent on a prime coat just makes it penetrate better. You want the wood as dry as possible, so that it's finished shrinking and so there will be a minimal amount of water adsorbed onto the cell walls to repel the oil. Pigment is detrimental to penetration because the pigment particles are too big to soak through the cells of the wood. If you don't need it for hiding purposes, best to leave it out. Fast-drying solvents like xylene (Kilz) are convenient and allow re-coating sooner, but slow drying solvents like stoddard oil give the vehicle more time to penetrate deeper.

Water-based coatings are useless because they are emulsions of paint droplets suspended in water (like butterfat in milk) and they don't penetrate at all. The droplets sit on the surface and then merge with each other as the water evaporates. Many urethanes have good wear resistance and UV resistance but lousy penetrating power. I have yet to find anything that beats a traditional oil (soy/tung/linseed) based paint with a petroleum based solvent. A little extra paint thinner in the first coat to make it soak in even deeper can't hurt. You're not concerned about shrinkage or strength within the primer coat because all the "buildup" should be happening inside the wood cells, where the wood provides mechanical support. What you're wanting to do is fill those cells, as deeply into the wood as possible, with insoluble resin to keep water out of them forever. The end grain, drilled holes, and rough surfaces are vital (keep painting more on those surfaces as long as it will suck it up, without letting it dry in the meantime) because the wood has natural tubes in it that will suck the oil way up inside the wood.

When you have bare, fresh quality wood like this, now is the time to prime it the right way so it will last as long as possible. Do something quick and dirty and it will look fine right away but the paint will start peeling off before you know it, and then you can never re-do it the right way.



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