Finishing resinous and blotchy woods

Overview

Two separate problems get lumped together here. Blotching is uneven stain colour: Southwick, in Popular Woodworking, says pine, fir, birch, maple, poplar and cherry are known to be susceptible, because the wood is unevenly porous. It can come from obvious grain-direction changes or earlywood and latewood density differences, but there are also variations in absorbency "that are hard to detect and predict". Resin and pitch is a different failure: the USDA Wood Handbook says many pines contain pitch, which "must be hardened or set", and that coatings will not stop pitch bleed. Knots of almost all species contain enough oils and resins to discolour light-coloured paint. Blotching is cured by managing absorbency; pitch by sealing knots or setting the pitch by heat.

When to use

• Before staining pine, fir, birch, maple, poplar or cherry (Eastern white pine, Knotty pine, Hard maple, Yellow birch are covered in the species guide).
• Before finishing knotty softwood with any oil-based topcoat.
• Not for woods whose problem is oil, not resin: see Degreasing oily woods. For end grain that darkens see Finishing end grain.

Materials and tools

Dewaxed shellac (Dewaxed shellac); a stain controller (clear gel varnish, wood conditioner or washcoat, see Pre-stain conditioner and washcoat); gel stain (Gel stain); dye (Water-soluble dye stain, Alcohol and oil-soluble dye stain); mineral spirits for a test wipe; scrap offcuts of the same boards.

Steps

1. Test first. Southwick says wetting a board with paint thinner "will sometimes reveal darker areas" that would blotch (How to Control Blotching). On cherry he says to find out how your boards will finish, which is easier with surfaced wood than rough lumber.
2. Choose a lighter stain colour if you can. Southwick adds that a light golden colour blotches less than dark tones, and that stains containing black or dark pigment, even in small amounts, usually produce the worst blotching. Where possible skip staining or choose a species that does not blotch.
3. Use a stain controller. The article's favourite is clear gel varnish: thick, so it fills porous surfaces better than thin conditioners. Applied wet and immediately followed by oil stain, or thinned and allowed to dry before any stain or dye. Sealing reduces blotching but also reduces colour; Southwick's estimate is that around 20 to 40 per cent sealed is a good balance (Sanding sealer (and vinyl sealer)).
4. Give conditioner time. Flexner says can instructions (stain within two hours) are unreliable and that thinned varnish needs at least six hours to dry fully before it works (Pre-stain conditioner and washcoat).
5. Pine process. Johnson's finish for pine (How to Stain Pine): apply water-based conditioner twice, then dye, then a coat of dewaxed shellac, then glaze (Water-based pigment stain, Water-soluble dye stain, Alcohol and oil-soluble dye stain). He uses shellac to keep pitch sealed; without it pitch bleeds into oil finishes and leaves fissures or shiny, tacky spots near knots.
6. Seal knots. Flexner lists shellac as the barrier for pine knot resin; the Wood Handbook adds that if pitch is already bleeding you can set it by getting it hotter than it will see in service for a day or two, then scrape, sand and wipe with a very small amount of paint thinner. Heat guns or hair dryers need care not to start a fire.
7. Loose or cracked knots. Dribble epoxy into gaps, tape the back, and sand flush; clear epoxy transmits the knot's dark colour (Repairing defects before finish).
8. Factory look on cherry and birch. Factories use glazes and sprayed toners applied over a sealed surface, putting most of the colour in the finish rather than the wood, so a stain and clear coat alone will not copy them (Spraying with HVLP).

Common problems

• Dark patches after staining. Hard to fix: bleach usually fails and sanding away the dark patches "can mean removing a lot of wood" (Southwick). Prevention is the plan.
• Black pigment in stain. It causes blotching even with a controller; check labels.
• Pale conditioned wood. Over-sealing cuts colour; use a thinner coat or switch to gel or dye.
• Cherry blotches without any stain. Flexner: most boards blotch when stained and also when finished clear; time darkens the wood to a rust red, and a cherry dye gets there faster but the wood continues to darken below it (Shellac).
• Blue stain in maple. Dresdner says maple is prone to a blue stain caused by mold during drying, which a strong 35 per cent peroxide solution (the "B" part of a two-part wood bleach) can usually remove; it is very caustic, so wear goggles, gloves and an apron.
• Pitch bleeding months later. The Wood Handbook says pitch exudation goes away naturally as the turpentine evaporates; scrape and sand.

Safety and environment

Shellac solvent (denatured alcohol) and mineral spirits are flammable and need ventilation. Rags with oil stain or gel varnish can combust spontaneously (Oily rag safety). Heating pitch with a heat gun or hair dryer is a fire risk.

Related

• Pre-stain conditioner and washcoat, Gel stain, Water-soluble dye stain, Alcohol and oil-soluble dye stain, Water-based pigment stain, Oil-based pigment stain
• Dewaxed shellac, Shellac, Finishing end grain, Degreasing oily woods
• Repairing defects before finish, Wipe-on application, Outdoor finishing