Avoid (or fix) Blotchy Stain
Clean up with a washcoat
Whether you use a store-bought wood conditioner or make your own from varnish and mineral spirits (more on that later), these so-called "washcoats" work the same: Solvent carries a small amount of varnish extra-deep into the absorbent, blotch-prone areas, partially sealing them. This equalizes the absorbency so liquid stain penetrates more evenly without blotching (photo - right).
You can use an oil-based washcoat beneath oil-based or water-based stain (photo - bottom right) once it dries thoroughly. To create your own, mix two parts of the varnish you'll use as a top coat with eight parts mineral spirits. However, don't attempt to thin water-based finish to make your own washcoat. Stick with the store-bought types.
Now the downsides: Partially filling the pores with a washcoat leaves fewer places where stain pigments can catch. That produces a lighter color than on wood with no washcoat. Washcoats also require some experimentation to prevent blotching while still coloring the wood.
Apply the washcoat generously to a test board until the spongy areas and end grain become saturated. When these areas stop pulling in liquid, wipe the surface thoroughly to remove the excess. Allow an oil-based washcoat to dry overnight and water-based washcoat to dry for three hours.
Conditioner labels may say you can apply stain sooner than that, but resist the temptation. You'll risk dissolving the washcoat with the solvent in the stain. If blotches appear, gradually add varnish to the homemade mix or apply additional coats of conditioner until it blocks the blotch. Don't exceed one part varnish to two parts mineral spirits.
Lightly sand the wash-coated surface using the same grit you used on the unfinished wood. Then stain according to the manufacturer's instructions.
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