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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.
Still Learning,
Allan Hill
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"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.