Polyurethane Over Boiled Linseed Oil


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I've searched the forums and can't seem to find anything on this topic.  I read that shellac works well over the top of BLO, but I haven't read anything about polyurethane.

 

The project I'm thinking of trying this on is very small.  It's a wooden bottle opener.  I apply the BLO on the lathe and am wondering if polyurethane will work over the top of the cured BLO.  I'm looking for the added texture and look of the oil finish but I also want the protection of the poly.  I've heard that it's a good idea to let it dry for about a week.

 

Has anyone tried this before?  Good results?

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Yes, assuming you are using an oil-based polyurethane they are perfectly compatible.  This will work fine.  Next time you could probably even mix 50/50 poly and BLO when you apply it on the lathe.  You will get the look and protection you want all in one step.  

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Not an expert on finishes so I'm sure I'll be corrected if I get this wrong but I believe if you're going to use a hard film finish over an oil you need to make sure the oil is thoroughly done drying or you're going to get hazing and spots under the film finish from the oil off-gassing and weeping.  This can sometimes take weeks and I think I've heard of the rare instance in which it took longer.

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yes you do want to make sure the BLO is cured. OP said the BLO was cured so i took his word for it. Honestly I have put poly (and shellac) on top of BLO the next day and it was fine. That was before I "knew any better". But I also now know that poly on top of BLO does not look much different than poly on top of bare wood. So if I am using poly I skip the BLO.

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Thanks for the information guys, I really appreciate it.  Byrdie, was it possibly raw linseed oil?  I've heard of the taking a very long time to cure, but never the boiled stuff.  Mr. Woodsap, I'm going to give that 50/50 thing a try this week, that sounds pretty cool.  If plain poly is going to have the same effect, it would have to be brush-on, wouldn't it?  Would a spray can have the same penetration?

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To my knowledge oil finishes, in general, take longer to cure and ones that are absorbed take even longer.

 

I'm not a finishing expert and I don't even play one on TV.  But I do tend to leave my finishes cure a bit longer than the suggested time just as a matter of course.

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  • 5 years later...
  • 8 months later...

I too have used BLO with an oil urathane top coat, both plain and poly. First attempt, wore out a scraper. You have to let each coat of BLO dry for 24-48 hrs and if thick 4-5 days. I normally use 3 coats of BLO followed by 2-3 coats of urathane or poly. As my projects lately have used poplar (Tulip) this gives a great color and grain highlights. Especially as I sum bleach the heartwood to near white color.

Regards

Andy

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  • 1 month later...
On 6/11/2020 at 1:52 AM, Rio Vista Andy said:

I too have used BLO with an oil urathane top coat, both plain and poly. First attempt, wore out a scraper. You have to let each coat of BLO dry for 24-48 hrs and if thick 4-5 days. I normally use 3 coats of BLO followed by 2-3 coats of urathane or poly. As my projects lately have used poplar (Tulip) this gives a great color and grain highlights. Especially as I sum bleach the heartwood to near white color.

Regards

Andy

The issue is how long after the BLO has been applied (to pine) can an oil-based stain/dye and polyurethane sealer be applied?

It's exceedingly difficult to find related info. The answers I've seen claim that a sealer can be applied a few hours after the last layer of BLO has been rubbed off. Others say one must wait 30-45 days after the last layer of BLO has been rubbed off in order for the BLO to be cured.

Which is it?

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IMO, although the oil based stains and finishes should adhere to the BLO when it is dry to the touch, it would be better to wait until the BLO fully cures (no longer smells) so that it isn'trying to off-gas under the top coats.

But I question the advantage of using BLO in the first place. What benefit are you expecting from it?

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4 hours ago, Tpt life said:

Isn’t BLO one that may or may not come with dryers in it? That would explain widely varying advice. 

BLO used to be literally boiled. That causes changes that allow the oil polymerize much faster (still slowly) that if it were raw (nearly forever). I don't know where you'd get that stuff now.

But virtually all BLO available commercially contains dryers & I'm sure different formulations will behave a little differently

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16 hours ago, wtnhighlander said:

IMO, although the oil based stains and finishes should adhere to the BLO when it is dry to the touch, it would be better to wait until the BLO fully cures (no longer smells) so that it isn'trying to off-gas under the top coats.

But I question the advantage of using BLO in the first place. What benefit are you expecting from it?

I'm a first-time novice at this, but my intention is to use the BLO to protect the sealed wood from drying out too much and crumbling. I'm assuming (perhaps wrongly?) that ultra-dry wood is as bad as rotted wood.

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BLO does not "re-hydrate" dry wood, it only forms a mildly protective barrier against the penetration of dirt and water. It also adds a warm amber color that looks good on some species, and horrible on others. If the wood you are using is punky, so dry and rotted that it feels like a sponge, then infused resin stabilization is about the only way to make it suitable for building. Since this requires a vacuum chamber and an oven, stabilizing larger pieces isn't practical. If the wood you are using is hard enough to shape into joints and hold a screw, its fine. Attempting to saturate the wood with BLO will only result in a piece that 'weeps' oil for weeks or months. Voice of experience....

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What wtnhighlander said.

There is no surface coating that will prevent water from migrating back and forth between wood and air.  Many will slow that process down, and some more than others, but nothing stops that process.  Wood will eventually reach equilibrium moisture content, but even in the desert that isn't zero.  And wood does not crumble because it is dry-- the fibrous structure of wood is not dependent on water molecules.  So if you are encoutering crumbling there is another process, such as rot, at play.  But wood can and will crack if taken from wet to EMC too quickly.  

Hope that helps.

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6 hours ago, DrHillymyboy said:

I'm a first-time novice at this, but my intention is to use the BLO to protect the sealed wood from drying out too much and crumbling. I'm assuming (perhaps wrongly?) that ultra-dry wood is as bad as rotted wood.

Wood will dry out to the point where its moisture content reaches equilibrium with the humidity of its environment. Even if in a desert with 0% humidity the wood will not degrade as it looses moisture. What may happen is cracking or warping. Nothing can be put on it to prevent that from happening. Another popular misconception is that wood needs to be "nourished". It does not. Wood is already dead & needs no nourishment or moisturizing.

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