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From googling, I have learned that some people recommend wet sanding while you apply BLO (Boiled Linseed Oil). I've read a few different reasons as to why one might do this. Some say it helps fill pores, and this is what I'm interested in. The last piece I finished has some noticeable yellow streaks in pale sapwood due to pores being filled with finish. (EDIT: I've realized with help from Graphus that actually the yellow color in the pores is inherent in the wood. Possibly the finish added more yellow, and certainly it accentuated it, as a BLO finish is supposed to accentuate the colors in the wood, but the wood itself is yellow in those pores.) Not sure if it would have been the BLO or the wipe on poly that I'm seeing, but I'm hoping to avoid a similar issue this time, as this wood also has visible pores in pale areas.

Where I get confused is that some say you must first apply a coat of BLO with a brush or rag, to act as a sealer. And then only on the second application do you start the wet sanding. It seems to me that sealing it would, at least partially, fill in the pores with pure BLO rather than a slurry. Others do recommend starting the wet sanding process on the first coat. And of course none of them discuss the pros & cons of the 2 possibilities; they just say "do X" or "do Y".

So my question is as in the title. What are the pros & cons of the 2 different methods? I'm especially interested in knowing what is the harm, if any, of starting the wet sanding on the first application, but I expect there are angles this newbie hasn't thought of that I might should be aware of. And if you want to throw in any comments about how well wet sanding during application fills in pores and what the result will look like, I'll appreciate those as well.

I do have a bit of scrap I'm going to test on, and I guess I'll need to divide it into 3 sections (no wet sanding, wet sanding on first coat, and wet sanding only starting on second coat). I just feel like there are likely things I'll miss either due to the small surface of my test or the small time before I'll start finishing the actual project or just my inexperience, so I felt it was worthwhile to get some advice from people more experienced than I.

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    First off, congrats on the well-framed Q, and the obvious prior research. Asking for a clarification or a hidden detail relating to research you've already done is precisely the kind of thing that SE is for. "Where I get confused is that some say you must first apply a coat of BLO with a brush or rag, to act as a sealer." I think the key thing is the highlighted word :-) I'm certain this is nothing more than a personal preference that has become cemented over time as the way to do it (and might then possibly be justified or 'explained'). This is v. common, and not just in woodworking.
    – Graphus
    Aug 22, 2023 at 3:17
  • Could you post a link to a photo of the yellow streaks you describe in the pale sapwood? (It doesn't directly connect to your query here so it doesn't have to be edited into the body of the question.) Since in general this is not an issue that crops up generally I'm curious as to what you're seeing exactly, and how the wood species might be the main factor. I work with a fair bit of ash sapwood which starts out very pale indeed, particularly whitish, and ash as you probably know is another open-pored species.
    – Graphus
    Aug 22, 2023 at 3:32
  • "I'm certain this is nothing more than a personal preference that has become cemented over time as the way to do it (and might then possibly be justified or 'explained'). This is v. common, and not just in woodworking." @Graphus - Oh yes indeed it is. And it frustrates me in everything, not just woodworking. But it's especially bad with woodworking, for the moment, or anything else new, when I have very little experience and background knowledge to help me determine when it's just personal preference (or just plain wrong), and when it's something I need to pay attention to. Aug 22, 2023 at 3:43
  • @Graphus imgur.com/a/IDlpjmW These are not great pics but I think you can see what I'm talking about. I happened to have already taken these pics to show a friend. Both pics are of a piece of scrap wood that I'm sanding down to remove the finish so I can do my testing outlined in my question above. In the 2nd pic the finish is mostly removed except of course the pores are still there with the yellow filling. All of the wood in question is acacia. Aug 22, 2023 at 4:05
  • "Oh yes indeed it is. And it frustrates me in everything, not just woodworking." Me too! I'd best not get started on the cooking-related ones or we'll be here all day :-))
    – Graphus
    Aug 22, 2023 at 5:54

1 Answer 1

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And if you want to throw in any comments about how well wet sanding during application fills in pores and what the result will look like, I'll appreciate those as well.

I think this is the heart of the query, and the pros and cons you asked for will cover this which is why I've answered out of sequence and the pro/con list follows.

Just to note, most of the following applies equally to wet-sanding any finish into wood, not just boiled linseed oil.

Pros

  • "Saves a separate step."

You save a separate step, yay! But there's a reason I put this in quotes as the cons will cover.

Despite this being one of the most-cited reasons to wet-sand using finish, so it has to be included in the pros list, it's not as simple as implied/outright stated.

  • You don't need a separate product for this purpose.

There's evident benefit in keeping things as simple as possible in finishing, and if you can keep from cluttering the finish shelves or cabinet with yet another thing... that's a legitimate pro.

  • You are actually filling the grain.

Speaks for itself really, but there are caveats which are what the cons list is all about......

Cons

  • Hard to control if there's some sapwood present.

It is difficult to outright impossible to control how the grain gets filled with the sanding slurry where there is both sapwood and heartwood present; similar issue on any wood that exhibits a strong colour contrast, or where contrast has been artificially created using a mixture of species is used in the project.

There's a very similar issue just with plain sanding of wood that has sapwood present, or on mixed-species projects with some pale wood and some darker wood. So for example you certainly don't want walnut heartwood sanding dust in the much much paler sapwood. Or vice versa! In normal sanding it is just about possible to get such cross-contamination out, however when wet-sanding you're not dealing with a dry dust that can be blown or brushed out of the open grain......

  • While this does fill the grain it doesn't do it efficiently.

If you were grain filling more conventionally you'd fill the grain in, at most, two stages. You fill, get rid of excess filler, and you're done.

While with wet-sanding finish you do this what, three times at least? And some guides call for doing it many more times... so are we really saving a step? And we're certainly not saving time, which is the underlying implication. Any grain filler worth its salt is dry overnight. Some dry in under an hour!

In addition, each 'application' of grain filler using this method involves some sanding, which inevitably removes some of the filler already present. I'm sure anyone who has tried this has noticed the effect, particularly in shallow grain lines. So there can definitely be a bit of a two steps forward, one step back kind of thing going on.

  • Using BLO for this is hardly ideal.

BLO is fundamentally a slow-drying finish, despite the greatly sped up curing time compared to raw linseed oil. While there are some BLOs that cure much faster than others it's still not ideal for a couple of reasons.

BLO almost invariably has a distinct colour (although there's more colour variation than one might expect). This colouring of course gets imparted to the filled grain. Filling grain darker is often exactly what it desired, however it isn't always. And on paler species you might want it barely darker but there's no control over what you get when you fill the grain this way.

BLO never dries hard. It might feel hard on/in the wood, but we should be under no illusions that it is actually hard; that's mostly the wood we're feeling. Once fully cured (which takes about a month!) BLO is still a gelled solid, not a hard substance, so the filled grain is not nearly as hard as the surrounding wood. (Although to be fair, this is also true of nearly every commercial grain-filling product, past and present.)

Conclusion
There's obviously some benefit to the basic technique in certain situations. And like with all finishing how much you like doing something — the innate appeal of the process — is fundamental to many people's finishing regimes.

But BLO is not the thing to do it with.


So, alternatives?

  • Actual pore fillers/grain fillers (both names for the same thing).

As mentioned above, these are generally far more efficient at doing what is desired — filling the grain better, can dry much faster, and potentially give better control over the colour of the grain fill.

  • Filling with the final finish itself.

Apply enough film finish and the grain gets filled in along the way. The process is slowed, because there's some amount of sanding back either in stages or in one step near or at the end, but it's a far simpler process with fewer gotchas.

And here's a key thing: it may be the best way to highlight the real beauty of the wood, which is what we're really after in all of this.

Any opaque grain-filling material, including almost all conventional pore-filler products, is just like wood filler in being a dull or 'dead' material that doesn't catch the light. A film finish on the other hand is a transparent material which you see right through and can transmit light in both directions, both into and back out of the wood. So the result can be a noticeably "clearer" or "brighter" surface.

  • Don't seek to fill the grain at all.

There's no rule that says one has to fill the grain, especially today when people have a genuine appreciation for wood projects showing off the original character of the wood used, flaws and all. And there's a general liking for a close-to-the wood result, which is counter to the whole idea of grain filling.

It's a matter of taste as much as anything whether one should partially or completely fill the grain of open-pored species such as oak, walnut, sapele. It absolutely has its place but for many common projects it's simply not needed, and no particular attention need be paid to whether the grain gets filled or not (as long as it's fairly uniform).

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  • Thank you. Just what I was looking for. I knew there were commercial pore fillers but didn't want to go with that due to extra expense & one more container to store, but also partly because I assumed it would create that "dead" look, which I knew I did not want. But when I read about the wet sanding BLO as a way to fill pores, it sounded great. Your answer helps me see it's not likely to be the ideal solution either. Although, one of the "cons"- the heartwood getting into the sapwood pores, is something I'd prefer over the yellow lines. So I'll likely still test it and see what happens. Aug 22, 2023 at 18:22
  • I'm excited about the idea of filling in the pores with poly. I'm imagining applying a few coats of it on just the sapwood (I only object to the yellow lines against the almost white; the pores in the heartwood are fine), then sanding it back until I've got everything except the pores back to bare wood to begin the BLO followed by wipe on poly routine. Is this realistic? Or will I just open up more pores before I can sand enough of the poly off to be sure the BLO will be able to work its magic? I love what the BLO does to the acacia and maintaining that look is my top priority. Aug 22, 2023 at 18:37
  • It's possible you could successfully fill the sapwood & then fill the heartwood by wet sanding, but why bother? Why not just do the whole surface the same way while you're at it? After messing about with BLO in many ways for some years I would suggest using linseed oil for the thing it's best at, grain enhancement, and nothing else. It really has little to offer us other than that, given all the downsides & that other finishes are vastly superior to it in every other way. To boil it down here's what I'd do (and it's how I finish most things where I really want to maximise the look [contd]
    – Graphus
    Aug 22, 2023 at 20:09
  • ... of the wood). I'd apply the BLO to 'pop the grain', then start applying poly and keep going. To fill the pores I'd just apply more poly, sanding back periodically (most do it after every coat but that's not strictly necessary) or sanding back in a single step at the end, depending on my mood and certain details of the thing I was working on.
    – Graphus
    Aug 22, 2023 at 20:10
  • I should probably emphasise that I didn't mean that filled pores make all of the wood look dead, not at all, it's only a bad pore-filling job that has this problem (excess filler wasn't conscientiously removed from the surface). It's just that there can be a better result the other way. If you're fussy (attentive) you'll notice the improvement, and more in some species more than others. But you should expect that not everyone can really appreciate this sort of thing and, like a lot of stuff we woodworkers sweat over, it might be only us and our ilk who would notice it a lot of the time :-)
    – Graphus
    Aug 22, 2023 at 20:15

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