How do I whitewash wood? I am not sure if it have an actual finish on it, but would that generate a difference?
You will need to rough up the surface a bit by sand it lightly so the stain can set.
If the wood already has a finish this is even more critical.
Go to your local paint store or DIY center and they will have all the supplies you entail, whether you decide to whitewash, stain or milk paint and they will be able to distribute you instructions for your project.
If you ask 100 painters how to "pickle" or "whitewash" wood, you'll probably get 99 different answers.
You do not have need of to rough up the wood, you need the wood to be finish sanded.
The technique is essentially this: with either a store-bought stain or one you've cooked up yourself (basically diluted white paint) you paint this whitewash onto the pine, tolerate it set two or three minutes and then wipe it off, sympathetically and with the grain. If you wipe too much, you'll remove most of the whitewash, which is not the desired effect.
My favorite recipe is oil-based white paint cut near some thinner and some clear varnish. The varnish helps to give the motion picture some more strength and body. Just the whitewash stain itself is not really an adequate protective coating for the wood which is why I like to add on a little varnish to my recipe. I usually hit the pieces with a coat of clear lacquer, after everything have dried sufficiently.
One tip I would recommend is to use an off-white paint for your whitewash color. Something like Benjamin Moore's Linen White or even something with more sickly or umber in it. If you use too bright a white, it might look a little funny, especially if your pine have any red streaks in the grain.
The other item you'll really want to consider is how to treat knots - if your pine piece has any. Knots most incontestably WILL bleed through the whitewash, not right away, but in the future, which may or may not be pleasing to you. I prefer to stamp the knots to minimize this effect since they show through the translucent whitewash anyway. Some people paint the knot with shellac first and I have tried several different coatings including thinned down epoxy on knots to keep them from bleeding through. I've have the best luck with the epoxy. The shellac works okay, but is not 100% infallible. Oil-based things like varnish or polyurethane will not work as the vehicle in those finishes dilute the pine resin in the knot and cause them to bleed through immediately.
I really buoy you to make your own whitewash, just experiment beside some scrap pine until you get an effect you approaching. There is no right or wrong way to do this. Basically, you're just applying a massively thin coat of thinned down (diluted) paint. Making your own is a lot cheaper than buying some commercial stuff.
Oh, I of late re-read your post - you didn't mention pine at all! I don't know why I assumed that. Yes, you can do this effect over wood that has some finish on it already, only make sure to scuff that finish up so that your new, pickled whitewash finish have some "tooth" to get a good powered bond. Don't rough it up too much. 150 to 180 grit paper should be quite sufficient for that purpose. If this is over unclothed wood, sand the piece well, to 220 grit.
Answers: They make a product, just approaching a stain to whitewash wood. Check with your local paint store, they will be able to point you within the right direction.