Pickling and whitewashing are two words for the same procedure -- preserving wood by coating it with lime just as Tom Sawyer did in Mark Twain's novel. Modern finishers use paint or stain instead of lime to achieve a whitewashed look, applying it to both finished and unfinished wood. When the wood is bare, the effect is deeper and more authentic, but whitewashing over an existing finish avoids the need for stripping.
To Strip or Not to Strip
If your cabinets have never been finished, you need only sand the wood with 120-grit sandpaper to open the grain. If you're whitewashing existing cabinets, decide whether it's worth the effort and mess to strip the finish. Whitewashing stains and paint soak deeper into bare wood and provide better coverage, but you can get fairly good coverage on finished wood if you clean it with trisodium phosphate and water to remove oils and grease and sand the finish with 120-grit sandpaper. You don't need to remove the finish -- just wear it thin and scuff it, so the whitewashing material can adhere and hide or blend with the existing stain.
The best whitewashing paint is a flat paint with a high solids content. You can buy heavily pigmented whitewashing and pickling stains, or you can simply use white primer. If you're whitewashing unfinished or stripped knotty pine, oak or another wood with an uneven grain texture, consider shellac-based wood primer. It covers evenly and seals the grain to prevent the natural oils from knots from bleeding through. Primer also works well on finished and scuffed wood, but a whitewashing stain works just as well. If your goal is an off-white pickling effect, use either a pre-colored stain or add some pigment to white primer.
Applying the Whitewash
A whitewash doesn't have to penetrate deeply to look good, but it needs to be uniform. Before you whitewash unfinished wood, seal the grain with wood conditioner to prevent blotchiness around knots and seams. The basic procedure for applying the whitewash is the same for both finished and unfinished wood. Brush the paint or stain on in even strokes, going with the grain of the wood; then wipe the surface with a clean rag -- also following the grain. Both the amount of time you wait before wiping and the amount of pressure you use affect the opacity of the stain, so you may want to perfect your technique on a test surface first. Always wear a respirator when painting or staining, and keep the windows open for ventilation.
Protect the Finish
Whether you use primer or whitewashing stain, it will dry to an unappealing flat finish that won't stand up to much abuse. Protect the finish and give it a sheen with one or two coats of clear polyurethane. Brush the first coat on directly over the stain; wait for it to dry; then scuff it lightly with 220-grit sandpaper and apply a second coat. If you're after a weathered, antique effect, try scuffing the stain in strategic places before you apply the clear finish. You can also add antiquing effects by using an off-white stain as a glaze. Brush it over the first coat of finish; wipe off most of it; then apply the second coat of finish.