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.
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......
- 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.)
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.
- 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).