I'm currently working on a entry bench in black walnut. I'm wondering if I should raise the grain before finishing my bench or not. In the past, I haven't done so. I've heard that it's better to raise the grain. Yet, I don't know for what reasons.
So, what is the purpose or advantage of raising the grain before finishing a furniture piece with a water based stain or varnish?
You want to raise the grain before using water-based finishes, as those will also raise the grain, but you won't be able to adequately scrape/sand off the raised fibers. I'd recommend using distilled water, as it doesn't have any minerals that can discolor some kinds of wood. Don't flood the surface, but wipe it on with a clean cloth and allow it to dry fully. Scrape or sand away the raised grain gently, then repeat as necessary until the wood remains smooth after wiping on water.
Water swells the wood fibers and they will plastically (permanently) deform. This leaves a rougher surface than what was prepared by sanding or scraping. Sanding after staining typically causes the color to be more uneven, and needing another coat of stain to even out more. Pre raising the grain with plain water and lightly sanding with ~320 grit to remove the raised grain (after drying) will allow a WB stain to be applied and have a smooth surface after drying to top coat. If no stain or dye is used, then there is really no advantage to pre grain raising for a WB topcoat. The 1st topcoat can be lightly sanded instead.
It is best to flood the surface until no more water is absorbed (a spray bottle and foam brush work well), and then wipe off and allow to dry. This ensures that when the WB stain/dye is applied the level of absorption by the wood fibers does not exceed the previous wetting, ensuring no further grain raising occurs. I will typically raise the grain on softwoods twice, flooding the surface the 1st time, sanding, then wetting more lightly the 2nd time. Softwoods soak up enough water to raise large areas of grain, and the swelled fibers in those areas are removed by sanding flat. The 2nd application addresses these areas.
Good answers, but nobody seems to have mentioned yet that some timber types are very prone to grain raising, and some really are not.
I have never "raised the grain" by dampening as a separate process, but I have seen the effects of this raising after applying a water-based base coat. From my (admittedly limited) experience, denser hardwoods seem to be less prone to the grain raising or "fluffing" up, but this may not be true in all cases.
Also, it actually depends on what kind of finish you want. See this link: http://www.wood-finishes-direct.com/info/faq#should-i-worry-about-raising-the-wood-grain
Is grain raising (with water based products) something to be concerned about?
In short - no, but it is a question of personal taste. The grain doesn't feel rough to the touch, it's more of a textured feel. In other words the pattern of the wood can be felt through the wood finish. Some customers really like to be able to feel the wood. If several coats of varnish are being applied then grain raising is not really an issue because the varnish tends to seal over the grain that has been raised, thus creating a smooth surface.
It's also worth considering that if you're going to be applying a finish with more than one coat, it's often the case that the timber will be sanded between coats anyway. With Meranti timber (which is a somewhat light hardwood which is quite prone to fluffing/raising grain) we would apply a base coat, sand it to key the finish (which also has the effect of flatting the grain) and then apply a topcoat, and be left with a nice smooth finish. It does mean that the denibbing/keying phase of the finishing process takes a bit more effort, but the need for raising the grain as a separate process is removed.
I wouldn't say "don't bother", but I would think about the type of finish you're trying to achieve, and whether it is worth it on the particular type of wood you're using.
As with most types of finishing, I would suggest you try it with and without this process on a scrap piece of timber and see the effects for yourself.
Raising the grain not only helps to bring out fibers that might swell when applying the actual finish, but saturating the grain beforehand with a wood conditioner will help to make the grain "pop out" (be more visually appealing) which when you're trying for a beautiful finish, is something that you'd normally want.
