Polyurethane over Boiled Linseed Oil Timing? -- Butcher Block Tabletop

Question

Short version

If I'm using a few coats of boiled linseed oil (BLO) followed by a few coats of polyurethane, what should the timing schedule be? For my specific use case, this will be a large butcher block tabletop in Sapele wood.

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Longer version

I'm looking to finish a large butcher block tabletop in Sapele wood that will be used for a writing/computer desk. According to the manufacturer of the butcher block (Hardwood Reflections), the wood "MUST be sealed or finished on all surfaces" immediately after opening the interior plastic wrap it comes in. According to them, this is to prevent the wood from drying and cracking. When I was first looking into it, it seemed to me that the manufacturer was specifically implying it needed to be oiled (they mention mineral oil repeatedly, but I definitely want to avoid that because this will be for a desk), but further reading on their site seems to indicate elsewhere that a poly would be fine (in a FAQ: "we highly recommend sealing within three (3) days of unpacking it with either a urethane or an oil finish"). With that in mind, here are all the wrinkles of my specific use-case that might actually wind up invalidating the original question and implying I'd be better off going in a different direction.

• I have about a month to devote to this project, with an hour or two every few days available. While I'm fine to put in a decent chunk of time, will I see a meaningful difference doing BLO followed by (oil-based) poly? Or will it not really make much of a difference in terms of look if I just jump right into using an oil-based poly? I know the BLO will impart a slight ambering, which is a slight plus for me, but not a huge deal.
• I know "drying" times vary based on environment -- this project will be taking place in May in Southern California, working in a garage, so pretty warm (80-90F) and fairly dry.
• Would there be a significant benefit doing this with tung oil instead of BLO? Like I said, I'm only slightly positive on the ambering, a lot of the reason I'm picking BLO is because it's easier to buy than tung oil and seems that it would cure faster.

Answer 1

I have about a month to devote to this project, with an hour or two every few days available.

So this is a good-news-up-front deal: that's ample time to do a number of different finishing regimes, and with the fastest of them (see bottom) most of the month can be devoted to just waiting out the cure period after the final coat has gone on.

While I'm fine to put in a decent chunk of time, will I see a meaningful difference doing BLO followed by (oil-based) poly? Or will it not really make much of a difference in terms of look if I just jump right into using an oil-based poly?

As I say in my Comment above, there's often little to no benefit in applying oil first before poly. Where applying oil first does provide a visual gain — greater "grain pop" and/or improvement in chatoyance — the difference can be slight enough that it's very much an individual call whether it's worth the extra time and effort required.

It's mostly about the species you're working with, but it can come down to the individual piece of wood you're dealing with sometimes (likely to be irrelevant here given that you're finishing something made from so many different pieces¹).

I know the BLO will impart a slight ambering, which is a slight plus for me, but not a huge deal.

So does oil-based varnish. Although there is sometimes a marked difference in the colour of the two liquids, there's usually no detectable difference once on the wood.

It seems surprising but it shouldn't be when one bears in mind that you get a very similar colour change if you just wet wood down with water or mineral spirits, both of which are colourless of course. Wetting down is sometimes used to 'preview' the finished look (and it's a great way to check for incompletely wiped-away glue, and missed glue spots).

I know "drying" times vary based on environment -- this project will be taking place in May in Southern California, working in a garage, so pretty warm (80-90F) and fairly dry.

I think you should have zero worries about drying time (or as you accurately refer to it, 'drying' time) at those temps unless your humidity was very high, which it seems is not the case.

a lot of the reason I'm picking BLO is because it's easier to buy than tung oil and seems that it would cure faster.

If it's pure tung oil that you mean, then yes usually² BLO cures significantly faster (although curiously this doesn't make for a faster finishing regime if doing a pure oil finish, at least the traditional way, LOL).

So, what to finish with
Obviously you have many many options here. You could use just oil. You could oil first, then start applying your varnish. You could use the polyurethane by itself.

You can even blend your BLO and varnish together, then thin the mixture further, to make "California oil" so to speak³ :-)

But I would suggest going straight to poly makes the most sense for you. Regardless if the surface were not that large I would recommend thinning the finish to turn it into wiping varnish because even for first-timers it makes varnishing virtually foolproof; by comparison brushing on varnish is considered something only pros can do reliably.

It's important to wait as long as possible after finishing something before putting it into service. At least a fortnight and ideally a month (or longer if chance allows). So in your case if finishing takes under week you can spend most of the month just admiring your handiwork and waiting patiently for the finish to toughen up so it provides max protection.

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¹ So while you might have individual strips (or staves in UK parlance) which benefit it could easily be flanked on both sides by pieces where there it makes no difference.

² Every Pure Tung Oil or Boiled Linseed Oil on the market does not have the same stuff inside the container, and there can be some quite marked differences across the board in 'drying' times (and, incidentally, in the look they can achieve, especially in the shorter term).

³ This is pretty much what commercial "Danish oil" is, minus some added drying agents..... and also "Tung Oil Finish" which is just a marketing name (and usually contains not a trace of tung oil).

Answer 2

I have used BLO under water-based poly in the past, to get the coloring effect of the oil without using an oil-based poly (mainly due to that being on hand for the project). The best recommendation I could find is wait until the BLO smell is gone completely before applying the poly. For me it took about 2 or 3 weeks.

But if you are already planning to use an oil-based poly, I don't really see a benefit of the BLO underneath. You can give it a try on a piece of scrap wood of the same type as the project - put BLO on one half of it and leave the other half bare. Wait a few days and then apply the oil-based poly. That should give you an indication of the color you will get, if not perfectly replicate the final result (due to potential adhesion problems that won't give the same protection from the poly as waiting for the BLO to fully cure).

Finally, shellac is often recommended as an intermediate layer between different finishes. I'm not sure if shellac would help with the cure time of the BLO, though, but would probably help the poly adhere better to the cured BLO.