Sealers, Sanding Sealers, Stain Controllers, and Wood Conditioners

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I have grouped these products together because many products tend to serve more than one purpose within these categories.

In general terms, a sealer is anything that seals the wood's surface, plain and simple.  Stain controller and wood conditioner are specialized semi-sealers: they partly seal the wood, perhaps only temporarily, to control how the wood absorbs a stain.  Now for the obligatory safety lecture, then let's get down to business:

Safety First!
Safety in the workshop is YOUR RESPONSIBILITY alone; I make no warranty as to the safety of any technique or tool shown or described on this site.  Before beginning any project, you must understand woodshop safety, know how to safely operate any machinery that is to be used in the project, and understand the safe use and any potential safety hazards involved in the use of all materials to be used in the project.  See the General Safety Notice and the Chemical Safety Note for additional information.

Sealer

A sealer is anything that seals the wood surface, usually by leaving a very thin film of varnish. The first coat of any varnish, shellac or lacquer acts as a sealer by sealing the wood's surface, but there are also purpose-made products for this job.  Sealer is like the clear version of a primer.  It seals the wood surface, allowing the next coat to flow out and build a film on the wood's surface, rather than soaking in.  A sealer may also be used to form a barrier between the wood and a coat of finish or between coats of different finishes or stains.  Oily woods, such as rosewoods and teak, contain oils that prevent the proper drying of certain finishes such as polyurethane.  A sealer made of shellac forms a barrier against the oils, allowing polyurethane to dry and cure normally.  Shellac can also be used to seal resinous woods such as knotty pine, so they can be painted.  Finally, a sealer is often used over a  water-borne dye, to prevent it leaching out of the wood and into a water-borne topcoat.

Sanding Sealer

A sanding sealer is a sealer with additives that soften it, making it much easier to sand the surface, without clogging the paper as quickly.  Sanding sealer is often used with lacquer finishes, but waterborne sanding sealers are also useful, since they raise the grain and seal in one step.  Since sanding sealer seals the wood, it is not recommended for use when the wood will be stained.  There are also woodworkers who apply as many as twenty coats of sanding sealer in order to fill the grain of certain woods and provide a base for a glass-smooth finish.  While I do not personally subscribe to this practice, I prefer to stay out of the debate whether this is a better method than traditional grain fillers.  I will note, however, that sanding sealer is softer than the topcoat form of the same finish, and the sanding additives make it rather cloudy as well.

Pre-Stain Sealer

A pre-stain sealer is formulated specifically for partly sealing the wood so it will accept stain without blotching; not to be confused with a pre-stain conditioner.  Pre-stain sealers are basically thinned down sealers that don't completely seal the wood surface. The idea is to apply the sealer, which will naturally soak more deeply into end grain and porous spots on the wood's surface. Once the sealer is dry, the wood is sanded lightly. This cuts through some of the sealer on the very surface of the wood, allowing the stain to penetrate a little, but doesn't cut completely through where the sealer has soaked in more deeply (i.e., end grain).  Since the amount of sanding done affects the amount of sealer removed, and therefore the amount of stain absorbed, I prefer to use a stain controller, which offers a less hit-or-miss method of blotching control.
*Note: finish manufacturers are notoriously confusing in their product labeling, so the easy way to identify a pre-stain sealer versus a pre-stain conditioner is to read the directions.  If you let the product dry completely, it's a sealer.  If you have a drying period followed by a window of time during which the stain must be applied, you are looking at a conditioner.

Washcoat or Spit Coat

A wash coat does basically the same thing as a pre-stain sealer, but is typically home-made by making a 1/2 pound cut, dewaxed shellac.  This is then flooded onto the wood, and the excess wiped away quickly before it dries.

Wood Size

Size is a term used to refer to any filler used to prepare a surface (e.g., for paint) by filling and sealing it.  Wood size is typically hide glue thinned to a watery consistency.  It may be applied to exposed end grain areas prior to staining to mostly seal them, so the stain will make the end grain about the same color as the rest of the piece.

Stain Controller / Wood Conditioner

A stain controller or pre-stain conditioner is usually a specially formulated, slow drying version of the solvent and base for the stain with which it is meant to be used.  The wood is flooded with this clear liquid, thus quenching its initial "thirst."  The surface is wiped down to remove excess liquid, and then the stain is applied between the time the conditioner sets to a tacky consistency and the time it dries.  End grain and other areas that normally soak up a lot of stain will have already been saturated with the conditioner, so they absorb about as much stain as the face grain, and stay relatively light in color.  A conditioner is also useful when staining large pieces that need to be worked in sections, because it also slows the drying of the stain somewhat, allowing more working time.
*Note: finish manufacturers are notoriously confusing in their product labeling, so the easy way to identify a pre-stain sealer versus a pre-stain conditioner is to read the directions.  If you let the product dry completely, it's a sealer.  If you have a drying period followed by a window of time during which the stain must be applied, you are looking at a conditioner (a.k.a. stain controller).

References:

Complete Illustrated Guide To Finishing, Jeff Jewitt, Taunton Press, 2004

The New Wood Finishing Book, Michael Dresdner, Taunton Press, 1999