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chris kennedy
wood conditioning prior to painting

I am a house painter. I have recently been asked by a client to apply a solution of linseed oil and turpentine to the exterior wood (clapboard,trim,cornice etc.) before applying an alkyd primer and two top-coats.I have gotten mixed feedback as to the performance of the pre-paint conditioning step I have described. Any advice?

Re: wood conditioning prior to painting

I'm definitiely no expert, but it sounds wierd to me. Seems like the thinning agent would hurt the alkide's adhesion, but maybe this client is a "chemist" and knows it's right. I'd say this is more of a contractual question: do what the client asks but make sure it's in writing that you're doing what the "Client requested" and you can't be liable for the outcome or the long term performance of the coating from both a materials and workmanship standpoint. I could see this one get ugly if a problem surfaced. Good luck!

Re: wood conditioning prior to painting


Is the client requesting this BLO (boiled linseed oil) primer step because of a existing problem with paint adhesion? (peeling paint, perhaps) Or is this to be applied to new bare wood?

Please don't tell me he wants you to use raw linseed oil as that would be a really big no-no. :eek:

If it's because of an existing adhesion problem, then the culprit may well be the result of water vapor migration from the interior of the house condensing and wetting the claps........causing a peeling paint problem. Or there may be exterior flashing or roofing problems. If any of those is the case, then using oil or alkyd-based primer products won't resolve it and could actually exacerbate the problem in the instance of water vapor migration.

If this is all new wood and there is no vapor migration problem (known or anticipated), then using thinned BLO could be beneficial, albeit somewhat expensive. The reason that it could be beneficial is that BLO is a true oil (as opposed to an alkyd) and the size of the BLO molecules are small/tiny enough that they will actually penetrate into the wood fibers producing an integrated deep bond. Alkyd primers on the other hand, cannot penetrate and bond deeply in the wood fibers because the alkyd molecules are simply too large to penetrate. They basically just sit on the surface and rely entirely upon the binder/glue in the formula to keep them there. Thinning alkyd primers with mineral spirits or turps doesn't reduce the size of the molecules, so there won't be any real penetration benefit from doing so.

The reason that turpentine is the preferred thinner for this type of BLO application is that it evaporates much slower than mineral spirits/paint thinner and that extra evaporation time gives the BLO a prolonged time frame in which to penetrate the wood fibers more deeply. Once the turps is completely evaporated, it will take several days or more for the BLO to completely oxidize/set-up. (Temperature and humidity factors will alter that curing time, of course.) Consequently, if you do the BLO app, I'd recommend giving it plenty of time to dry/cure before applying the alkyd primer.

I think your client's theory/notion is that once the BLO has penetrated and bonded, then the alkyd primer can attach to that, yielding a result that will more deeply integrated/penetrated and hypothetically superior. Or maybe your client knows nothing of all this and is simply requesting it because "that's the way we always did it"......way back when.

I suspect you already know this stuff well, but will yammer on it a bit for those who may not. I personally wouldn't advise using an alkyd for the topcoats over the BLO and alkyd primer because today's alkyd paints dry harder and are less flexible than the "true oil" BLO based paints of yesteryear. Consequently, they can't flex as much when the wood shrinks and expands and so they fail/peel more readily/sooner. Flexibility/elasticity is an important quality to have in an exterior paint. Top quality waterborne topcoats are the way to go over the alkyd primer, IMO and IME. SW Duration is a favorite of mine these days and has been since I first used it about 12 (?) years back.

I guess the bottom line is that I can't say definitively whether there is or isn't any real benefit to using the BLO "pre-primer" app these days if you use a quality alkyd primer that has sufficient binder in it. It might be that the BLO would provide some water-repelling benefit if the paint degrades/wears out and the house isn't repainted in a timely fashion. IME, the performance of quality exterior alkyd-based primers (as compared to the long-oil primers of days gone by) has come a long way compared to 20 years back so that the potential benefit of using BLO becomes less of a factor. But....... I doubt there would be any problems caused by using it.............provided that it's given time to cure before proceeding. IMO, the last thing you'd want to do is rush the stages of the job and end up with uncured BLO under the waterborne topcoats as that could cause bonding and curing problems for them.

Re: wood conditioning prior to painting

This is not weird, it is just an old fashioned way to condition very weathered wood prior to painting. This would be used on a barn or house that hasn't been painted in decades. Perhaps your client has this condition and has heard about this "grandfather's" remedy.

Instead of this homemade concoction, you can also try Penetrol by the Flood company, which does the same thing and has a long track record. Just spray or brush onto the weathered building until the wood won't take anymore. Wait until dry and then go to primer and top coats. The Penetrol doesn't take as long to dry as the home made wood conditioner mentioned. Paint goes on smoother and stays on better after old really weathered wood is conditioned.

Do a test on one of the clapboards and see what you want to do.

Re: wood conditioning prior to painting

I would actually mix 4 parts primer, 1 part oil and 1 part turps and make a penetrating oil primer that way. This will need a second coat of primer, unadulterated, or maybe a dash of turps added instead of mineral spirits.

Re: wood conditioning prior to painting

If this is new construction, it sounds like overkill to me.

Homes in grandpa's day were devoid of insulation and vapor barriers. Do what you may, they usually ended up peeling, especially after several coats of vapor retarding oil paint. This constituted a vapor barrier on the exterior - right where you don't want it! Modern acrylic based housepaints allow water vapor to pass right on through.

I prefer an alkyd oil primer as opposed to an acrylic primer because it bonds well, sets the grain and seals in an tannins. A couple coats of acrylic paint presents a vapor permeable layer which sheds rain water. It is relatively color fast compared to oil products. It is elastomeric, moving with the wood as seasons change.

When putting solid hide wood stains on cedar siding, I would use the oil version of Cabot's as the primer. I would wait for the linseed based stain to dry about a week in summer, and then top coat with the same color of acrylic based Cabot's stain. This is admittedly contrary to Cabot's instructions, which call for a conventional primer first. I always had good results. since the majority of my customers were long time repeat customers, it behoved me to do the very best for there homes.

Re: wood conditioning prior to painting

An old thread, but g0ldhiller has it right. BLO is an excellent wood treatment, thinner helps it penetrate and dry in a controlled manner. It's what I use on gunstock finishing and it both hardens the surface and protects against moisture better than anything I know of. Even surface damage like scratching doesn't void the waterproofing characteristic since it penetrates well below the surface. I feel it is better than the usual primers, but it's costly and I can't say how much better. It's overkill for house painting IMHO unless specific circumstances call for it.


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