I know conditioning is an extra step that may just seem like a hassle, but depending on the longevity of the project, it's absolutely worth it. Since wood pores are irregular, a conditioner can be used to reduce the likelihood of blotchiness from the staining process. This is especially true of soft or porous woods.
If you are going with a stain, wood conditioner is a good idea. Poplar tends to stain quite blotchy. If you are using Tung oil (or linseed oil, or any other penetrating oil-based product) stay away from conditioners. For low-maintenance preservation a good polyurethane can be used without a conditioner.
Specifically designed to use before staining to help prevent streaking and blotching and help ensure a beautiful, even stain. Quickly dries in 15 minutes to minimize project time. Can be applied over any wood but is especially necessary when working with soft or porous woods like pine, alder, birch, and maple.
The conditioner works by penetrating and temporarily sealing the wood to even out the rate of absorption, thereby creating a much more uniform stain coat.
Bare wood has natural variance in the dryness of its surface. Drier spots on the piece of wood will absorb more stain, making them turn darker in color. This can give wood projects the dreaded splotchy finish.
If you are using an oil-based polyurethane, you should have no problems applying polyurethane on top of wood conditioner, given the wood conditioner has fully dried. If you're applying a water-based topcoat, apply Zinsser's Shellac Sealcoat to seal the wood first.
Wipe off excess with a clean, dry cloth. Apply stain over conditioner. Let Pre-Stain Conditioner dry at least 30 minutes, but no longer than 2 hours before applying stain, in ideal conditions: 70°F/20°C; 50% humidity.
Most manufacturers recommend applying the stain in a window between 15 minutes and two hours, but consult the can for specific dry times. Applying stain too soon may just dilute the color without preventing blotching, but waiting too long may prevent the stain from soaking in at all! Apply the stain.
A sanding sealer is not the same as a wood conditioner. Whereas a wood conditioner is a pre-stain treatment that reduces blotchiness when staining, a sanding sealer is applied only to bare wood that is not going to be stained.
To properly answer your question of “Do you have to seal stained wood?”, the answer is no. However, if you want to protect your wood and give it a polished finish, it's smart to do so. It's worth taking the extra steps and applying a wood stain sealer.
There are three types of clear topcoats used to seal natural wood or bleached furniture – polyurethane, polycrylic, and wax finishes. Each type of clear coat has different pros, cons, and projects that it's best suited for. Polyurethane is available in oil-based and water-based finishes.
When the moisture content of the wood is below about 25%, you can use the sealer, but if you haven't got a moisture meter that's not going to help much. Damp wood can be felt with the skin as a sensory organ; this is not widely known.
On oak and walnut, it depends. A pre-stain conditioner can limit absorption and therefore the shade of the stain - it may not be as dark. Pre-stain conditioning will even out the color tone across the piece, and this may or may not be desirable, i.e. for a more rustic look don't condition.
Water-based wood conditioner will cause the wood fibers to swell slightly. After the wood conditioner dries, lightly sand the wood with #180-grit sandpaper to smooth out the raised grain.
If you want a smoother, more consistent stain (not blotchy) in your finished piece, I would recommend using some type of wood conditioner, whether it's already in the stain, a pre-stain from the manufacturer or homemade, or water.
The order which you apply stains and sealants varies on what you're currently working with. For the best performance, we recommend stain first, seal second, and apply Advance Topcoat third. This will provide the best protection of your home.
No mineral spirits are not a good conditioner for wood. Solvents in general will dry wood out and break down any oils that are in the wood. This leads to decay of the wood.
My Secret for Smooth Staining
Once you're done sanding, make sure you've gotten rid of ALL the sanding dust before you do anything else. Use a good vacuum and then a tack rag to wipe the surface clean of any contaminants. The next step is the trick here. Wipe the surface thoroughly with a a wet cloth.
We always recommend two coats of stain for any wood project, but you should only apply as much stain as the wood can absorb. Extremely dense hardwoods may only be able to absorb one coat of wood stain. The general rule of thumb is to apply only as much deck stain as the wood can absorb.