Finish Systems for Dry Interiors

The Wood Handbook defines moisture-excluding effectiveness as a measure of a finish’s resistance to the diffusion of water vapour — its permeability — and adds the two sentences that matter most for a mountain house: no coating is impermeable, and a coating that excludes water vapour merely slows its absorption or desorption; it cannot change the equilibrium moisture content. The companion guide on altitude and moisture shows where that equilibrium sits in a heated Sierra interior. The finish does not move it. The finish only decides how long the wood takes to get there.

The Wood Handbook’s own test puts numbers on how long. On ponderosa pine, three coats of a two-component gloss polyurethane paint were 96 percent effective after one day, 86 percent after seven and 74 percent after fourteen. Three coats of an oil-modified urethane varnish ran 90, 64 and 44 percent; six coats, 93, 76 and 62. An aluminium-flake pigmented varnish reached 98, 91 and 84. Three coats of linseed oil ran 33 percent after a day, 2 percent after a week and zero after two. A single brushed coat of paraffin wax, which no kitchen will ever wear, managed 97, 82 and 69. The list of variables the Wood Handbook gives — film thickness, defects and voids in the film, pigment, resin, the vapour-pressure gradient across the film, and the length of exposure — is the list of reasons a good film finish buys weeks and an oil finish buys days.

For a second home the useful word in that list is gradient. A film slows the exchange in proportion to the difference in vapour pressure across it, which is largest the moment a cold house is brought up to heat. A good finish is therefore worth most exactly when the occupancy guide says the cabinetry is most at risk: during the abrupt transition, when it keeps the two faces of a door moving together. The Architectural Woodwork Standards make that a requirement rather than a preference — panel products and wood doors require balanced coats of finishing material for stability and to remain free of warp.

Conversion varnish — System 5 in the Architectural Woodwork Standards — is an acid-catalysed finish that crosslinks into a new material once the catalyst goes in. The standards rate it 4 for general durability, 5 for solvent, stain and heat resistance and 3 for repairability, with a composite score of 129. M.L. Campbell’s Krystal data sheet gives the schedule: 12.8 ounces of catalyst per gallon, a ten-to-one ratio, a twelve-hour pot life once mixed, dry to touch in 15 to 25 minutes, sanding in 30 to 45, stacking in 4 to 8 hours, two coats at 4 to 5 wet mils for a 4 to 5 mil dry film. It is the shop default for a dry, hard-used house because it is durable, it schedules within a working day, and it cures in a conditioned booth before the cabinetry ever sees the pass.

A two-component waterborne polyurethane is the alternative for owners who want the lowest solvent content, and the numbers are honest about the trade. The standards score a water-based polyurethane at 112 — below conversion varnish, well above a pre-catalysed lacquer at 99 — and M.L. Campbell’s Arroyo lists 137.5 grams of VOC per litre against 579 for its MagnaMax pre-catalysed lacquer. The Wood Handbook’s test gives a two-component polyurethane the best moisture-excluding result of any film in its table, 74 percent after fourteen days. The constraint is the cure: Arroyo is catalysed at 100 parts to 5, rested five to ten minutes to release the carbon dioxide the reaction produces, applied only above 65 degrees Fahrenheit, stack-dry in a minimum of ten hours, and asked to cure for seven to ten days before use in a harsh chemical environment. The standards add that waterborne coatings must be cured in a dehumidified atmosphere assisted by air movement. All of that is achievable in the Rocklin finishing room and none of it in a cold cabin in March.

Penetrating oil is the finish that reclaimed fir and some knotty alder kitchens are specified in for the look, and the look is real: oil leaves the surface of the wood as the surface, with no film to catch the light. The standards are equally real about the cost. System 6, synthetic penetrating oil, rates 1 for general durability, 1 for moisture resistance and 1 for solvent resistance, and 5 for repairability and finish flexibility — the exact inverse of conversion varnish. The Wood Handbook’s linseed-oil result, zero moisture exclusion after two weeks, is the physical reason: oil in the wood does not slow the exchange with a dry room in any lasting way. An oil-finished kitchen in a Sierra house is a kitchen whose wood arrives at the room’s equilibrium fastest, and whose owner has signed up to re-oil it.

The Architectural Woodwork Standards record their finish-performance baseline as 45 to 55 percent relative humidity at 70 to 80 degrees Fahrenheit, note that cure of finish systems has a wide range of variance, and state that waterborne coatings must be cured in a dehumidified atmosphere assisted by air movement. The manufacturers add floors: Arroyo will not go on below 65 degrees surface and air temperature; MagnaMax dries to touch in 15 to 25 minutes at 77 degrees and far faster when force-dried at 120. None of those conditions exist in a house that was at a winter setback last week and whose kitchen is open to the great room during demolition.

That is the practical reason PineWood finishes every cabinet, door, panel and edge in its Rocklin workshop and delivers it cured. The spray booth and finishing facility sit at the conditions the standards and the data sheets assume; the cabinetry crosses the Sierra as a finished product, is staged in the conditioned house as the logistics guide describes, and is touched up on site only where scribing has exposed raw wood. A finish that is dry when it arrives at the lake is also, in the case of a conversion varnish stacked in four to eight hours and a waterborne polyurethane stacked in ten, well into its cure — the companion guide on finish systems explains why dry and cured are different milestones, and why a two-component waterborne is still gaining chemical resistance for a week or more after it could be handled.

The one thing a site cannot be spared is the wood movement beneath the finish, which no coating stops and no coating causes. What the finishing room does is make sure that when the Sierra winter arrives and the cabinetry dries toward the room, every face of every component does it at the same rate, under a film that was built to its manufacturer’s thickness and cured under the conditions its rating was measured in. The rest is the construction.