Why does...

[titanxt]

Why does poly work when mixed with BLO, yet doesn't if applied a while later (prior to the BLO curing)??

Thank you!

Andy

[AHill]

You can. You just have to wait a long time for the BLO to fully cure. It takes 10 days to 2 weeks for BLO to cure on its own. If not fully cured, the solvents in the poly will react with the uncured BLO.

[titanxt]

Why isn't there the negative reaction when you mix BLO and poly and apply together in a mixture?

Anndy

[AHill]

titanxt said:

---

Why isn't there the negative reaction when you mix BLO and poly and apply together in a mixture?

Anndy

---

Paul has pretty much explained why. If you mix them all together uncured, they will cure in about the same time. It's when you don't let BLO fully cure, then add a finish on top that has solvents where things go wrong. Doesn't have to be poly. It could be any other finish with similar solvents than poly. One thing you're doing if you add a finish over uncured BLO is you are preventing oxygen in the air from completing the curing of BLO. When they are mixed together as liquids, both are intermingled and both the BLO and poly are exposed to the oxygen in the air that allows curing. This is my theory anyway. My primary expertise is not organic chemistry.

[Paul K. Murphy]

Quote:

---

My primary expertise is not organic chemistry.

---

Not mine either.
This isn't the first time this topic has come up. I think it's very important for anyone who would apply finish to their woodwork to clearly understand the three types of wood finishes. Each type behaves differently, has different requirements, qualities, and different virtues. The three types are:
1- Evaporative (e.g. shellac, nitrocellulose lacquer)
2- Reactive (e.g. various solvent based varnishes, BLO, various two part whatevers)
&
3- Coalescing (e.g. various waterborne whatevers)
They do not behave in like manner.

[Paul K. Murphy]

"Work", and, "doesn't" isn't very explicit language, but I think I know what you mean.
Both polyurethane varnish (not waterborne) and boiled linseed oil are reactive finishes with like solvents. That is why they can be mixed. They will cure in the same arc of time. Though their respective curing periods may differ, greatly or slightly as the case may be, the mixture will cure at the rate of the mixture.
Both of these reactive finishes react with atmospheric oxygen. Some reactives do not, but these ones do. If you apply BLO and later apply PV, as has been indicated above, the solvents in the varnish will react with the uncured BLO.
Don't do that.
Just to make the matter more clear, a reactive finish, upon curing, is a different substance than what it was when it began. BLO or PV on your cabinet is not the same substance it was when it was a liquid in the can. An evaporative finish, like shellac or nitrocellulose lacquer is the very same thing in both instances. This sameness is largely theoretical, but true all the same. I say theoretical, because you won't have any success trying to re liquefy it and put it back in the can. I say that it is true because it's true, and you can use that knowledge for practical purposes. If you have an evaporative finish that isn't behaving itself, you can dope a thin coat with whatever medicine will address your problem, rewet the surface, and the medicine will treat the original coat. Rewetting the existing coat (with evaporatives) really, truly rewets it. The original coat reverts to what it was in the can and reacts to the dope appropriately. A reactive finish has changed its very nature. When you introduce something that may have mixed with the wet stuff in the can, you may be introducing an incompatible contaminant to the (now) new substance.
One thing that can happen is the new, hot (with solvent) can scald the existing coat. A solvent may also intermingle with something poised to become a butterfly, having been a caterpillar. You will also see shrinkage rates differ between reactives that have been there a while, and "new guys."
Reactives are great for producing tough finishes, generally.
Evaporatives are great if you want to keep pushing it around like wet clay.
Try not to make one do what the other does so well.