Mixing liquids. Just as all finishing products that thin with water can be mixed, so can all finishing products that thin with mineral spirits (paint thinner). Here I’m adding some stain of one brand to some polyurethane of another to make a “varnish stain,” a stain that can be left a little thicker on the wood or over another coat of finish.
I’m sure you’ve come across cautions in woodworking books and magazines instructing you to “use a compatible product” – stain, filler, glaze, finish – and you’ve wondered, “What is compatible, and what isn’t?”
The phrase, “use a compatible,” is a “cover-my-behind” dodge used by authors who have little understanding of finishes. If you follow their procedures and then have problems, it must be your fault for using an “incompatible” product. The burden is on you to know what is compatible and what isn’t.
So what is compatible with what?
Three entirely different situations can be referred to by the word “compatible:”
• Mixing liquids with liquids;
• Applying stains, fillers, glazes and finishes; and
• Coating over an existing finished or painted surface.
As I explain each of these, you will see that the issue of compatibility has been greatly exaggerated. In most cases, it’s obvious which liquids mix. Almost any finishing product can be applied over any other as long as the previous is dry. And almost any finish can be applied over almost any old surface as long as it is clean and dull.
Most products you use in finishing (or painting) are water-based or mineral-spirits-based. All water-based mix successfully, and all mineral-spirits-based mix successfully. But the two cannot be mixed together.
Wax mixed with varnish works well only if you wipe off all the excess after each coat, just as with wax mixed with oil.
It’s easy to know when two products don’t mix: they separate. For this reason it’s wise to use a glass jar for mixing if you have any question, so you can see what’s happening.
Almost any finishing product – stain, filler, glaze, finish – can be applied successfully over any other finishing product, except wax (including residue wax from paint strippers), as long as that product is dry. This includes every finish over boiled linseed oil, and water-based finishes over oil stains. You might need to give the oil-based product several days or a week to dry in a warm room, but once dry every finish will bond fine without problems.
Think of painting a piece of furniture you finished several years earlier with oil. You wouldn’t hesitate using a water-based paint.
There are several fairly uncommon exceptions to this rule.