When putting multiple coats of stain on a piece of wood, should wood conditioner be applied between each coat or only before the first coat?
should wood conditioner be applied between each coat or only before the first coat?
Only before the first coat.
Pre-stain wood conditioners are to help (note: not solve) with blotching, which is the irregular absorption of stain into the wood.
Most woods have some variation in absorption from earlywood to latewood, but for blotch-prone woods the absorption is irregular even across those zones and it's this in particular that leads to the ugly results that pre-stain conditioners are intended to help with.
Before there were commercial products for this purpose craftsmen obviously had to contend with blotching. The wood might be sized, painted with a thin layer of dilute glue, or have a dilute layer of shellac or varnish applied, a little later diluted lacquer was used in exactly the same way. This is called a spitcoat or washcoat.
This should give some pointer to what commercial wood conditioners might be, and in fact many oil-based pre-stain conditioners are nothing more than thinned polyurethane varnish... which of course the woodworker can make for themselves at a fraction of the cost.
Here is a good example of the improvement that pre-stain conditioning can provide:
But unfortunately results are not always this good.
Since the commercial products are much the same as what was (and still is) used by craftsmen it begs the question: why were there results better? The main reason is that the application recommendations on commercial wood conditioners can be incorrect. The instructions on the tin may suggest you apply conditioner and then within a short period apply your stain, but this is wholly wrong.
I'm sure this is in part to make the product seem more user-friendly in this I-want-results-now era, but it does the purchaser no favours. If instead you do what craftsmen did, and still do, and apply your per-stain conditioner (or thinner shellac or varnish) and wait for it to fully dry, results will be far better and more consistent.
An alternate approach
These days many of us can take advantage of a newer type of colouring finish, gel stains.
Gel stain is essentially thicker coloured varnish, and because of its gelled consistency it is not readily absorbed into the wood fibres but sits on top as a coloured layer. And for this reason it nearly completely prevents blotching, even on woods particularly prone to it such as pine.