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Specifically I am wondering about polyurethane (Rustoleum Ultimate) over boiled linseed oil, but in general polyurethane or other top coats with similar chemistry over any drying oil.

Question is: Can I apply polyurethane over an oil before the oil completely cures or do I have to wait?

I looked up linseed oil on Wikipedia and it led me to drying oils. These oils polymerize by reacting with ambient O2. I also attempted to research the curing process of polyurethane but discovered there are many different types with many different uses, so it was difficult to find info. I think they cure by reacting with ambient moisture.

My knowledge of chemistry is too limited to be able to understand if polyurethane would allow O2 to pass through to react with the undercoating of oil as it cures.

The other random bit of info that I found is that Rustoleum Ultimate polyurethane primarily consists of:

Neither of which I understand. I reached the conclusion that they are moisture cured from this article although it concerned types of polyurethane used on metal. Not sure if it's the same as wood.

So, can I apply polyurethane before the BLO cures? Will it continue curing underneath? Or do I have to wait? If I do have to wait, what will happen if I don't? Is it different for an oil-based polyurethane (it looks like the Rustoleum one is water-based)?

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    I asked a similar question here: woodworking.stackexchange.com/questions/1496/… If you look at @Graphus answer he says "Regardless if it will, it's still a viable step if you want to do a fast finishing job: you can oil, and then shellac over the top of it without waiting at all for the oil to dry or cure. Then once the shellac has dried (wait about an hour if possible but 15-20 minutes can do it) you can go straight to your first coat of varnish. Using this method you can go from oiling through to 3 coats of varnish in a single day."
    – Doov
    Oct 20, 2015 at 17:46
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    Might be worth considering a dewaxed shellac as an interface between the two, which would allow you to go ahead and apply the poly without worrying about the oil drying.
    – Doov
    Oct 20, 2015 at 17:48
  • i use laquer immediately over oil stains on much of my wood turning. it dries in less than 15 minutes, I then return the items buffing them with 0000 steel wool - beautiful finishes!
    – kim
    May 25, 2019 at 13:38
  • It's unclear how this answers the specific question of applying one sort of finish over another.
    – user5572
    May 27, 2019 at 13:57
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    Single component polyurethane finishes are uralkyds, I believe. That is: oil modified polyurethanes, where the oil is the part that bonds. The polyurethane part is already polymerized (but obviously not very long, or it wouldn't be soluble in solvent).
    – piojo
    Apr 10, 2020 at 16:44

2 Answers 2

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Can I apply polyurethane over oil before oil cures?

Yes, broadly speaking you can do this and there is no real penalty. You do tend to see an increase in drying time, but contrary to many statement to the contrary online the oil will cure perfectly well under a coat of poly, because the varnish is not at all a perfect oxygen barrier.

However, polyurethane is by no means a single product and the above does not apply to Rust-Oleum's Ultimate as it's a waterbased poly! As a rule no waterbased product should ever be applied to an oily surface.

If I do have to wait, what will happen if I don't?

You'll have significant application difficulties, as you'd expect applying a water-borne product to an oily surface. The most significant effect can be an uncontrollable beading, which prevents any sort of even coverage.

If you wait for the oil to cure however you can successfully apply the Ultimate onto wood treated with BLO. This curing time is at minimum a couple of weeks, but many users prefer to wait a month or so to err on the side of caution.

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    I would be less concerned about the oil not curing due to oxygen lack (although it surely will take 20-30 times as long!) than about the PU drying and fastening properly. PU pulls water when drying, both from the air (on one end) and from the wood (on the other side). If the wood has an oil barrier, that means no water on that end. Also, the oil film may (indeed quite likely will) prevent the lipophob PU from properly "creeping" into each and evey slit and crack. Which will of course still result in a "closed" finish, but not one that is nearly as durable.
    – Damon
    Oct 19, 2015 at 17:57
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    @JasonC, "say, 48 hours of letting the oil sit a bit before applying would be a safe bet" That wouldn't be an unusual wait for the drying oil to begin to 'dry'. Most people I think wait the customary one day, as though they were then going to apply the next coat of oil, and others wait a full week.
    – Graphus
    Oct 20, 2015 at 9:46
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    @JasonC: Both, to different extents (less for oil-based ones since these are not "hostile" towards non-cured oil). The nice thing is that it is a total non-issue if you just wait a day or two, letting the oil cure and change its physical-chemical properties (assuming treated "varnish" type of oil, natural oil will take a week). Which is what I would now always do, after my experience with that garden table (it's mighty fine as garden table the way it is, and I'll leave it at that until it peels off... but a good finish sure is something different).
    – Damon
    Oct 21, 2015 at 11:26
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    @Damon Water might dewet on the surface of oil, that doesn't mean that the oil is impervious to molecular water, and it's possibly less impervious as a liquid (ie before polymerizing). Think of plant leaves, for example, that have a waxy/oily leaf coating. It helps keep them clean, but it still allows for transpiration. It might be a slower cure with the oil, but like graphus says, probably not much since the oil layer is so thin.
    – aaron
    Aug 1, 2016 at 12:27
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    @Graphus of course the heavily oiled cutting board gets water damage - wood is hydrophilic! Even saturated with oil, wood (at least the cellulose part - the lignin might not) will preferentially partition to water. Otherwise xylem and phloem couldnt transport water.
    – aaron
    Aug 1, 2016 at 12:31
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Can I apply polyurethane over an oil before the oil completely cures or do I have to wait?

You can, but you won't like the results.

Not allowing the oil beneath to cure fully will mean that part of the oil mixes with the solvent used to carry the polyurethane (assuming oil-based poly), leading to a splotchy finish. If you're using a water-based poly, it will bead up like water on an oil slick.

See this related Question.

Specifically I am wondering about polyurethane (Rustoleum Ultimate) over boiled linseed oil

If you like the subtle tan/yellow coloration BLO provides, an oil-based polyurethane should provide a similar tinting. Or, you could use a tinted stain/dye for the same coloration and then finish with poly. Since BLO has a widely-varying curing time (especially in colder weather), it's far more consistent to use a stain/dye for coloration.

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    Re. the poly over undried oil leading to a splotchy finish, I haven't noticed that in practice. I've used varnish (multiple types) over freshly-applied oil many times and never had this problem. Experimentally I've even applied it immediately after the oil, with the same result — no visible difference. What mostly goes unacknowledged is that the usual 'drying' time allowed for BLO before varnishing means that in fact almost all users apply it before the BLO has cured.
    – Graphus
    Oct 19, 2015 at 18:31
  • @grfrazee In your experience, have you witnessed poor finish from a drying oil being affected by the solvents in an oil based polyurethane? Are certain woods more susceptible to this?
    – Jason C
    Oct 20, 2015 at 3:39
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    @JasonC, When I was a teenager, I made a set of picture frames for my uncle out of select pine (very clear southern pine, I believe it was). Made the mistake of applying varnish shortly after applying stain, and it was terribly splotchy. I can't be certain if the wood itself contributed, but I know that now I always err on the side of caution for these things.
    – grfrazee
    Oct 20, 2015 at 11:51
  • In your response to the Rustoleum Ultimate you wrote about oil-based products, but did not address the fact that RU is water-based. I think that what you wrote is correct, but it did not answer the question.
    – Ast Pace
    Nov 29, 2015 at 6:24

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