Finishing

Wood Stain and Finish

  • All wooden things must be protected with a topcoat; aka finish. The finish protects the wood from the moisture present inside the typical house/building. Without a finish most wooden items will begin to deteriorate quickly. Plus, a finish improves the looks of wooden items.
  • Wood stains only color wood. Some exterior stains contain preservatives and topcoats, and will not be covered here. Stain is only needed if you wish to change the color of the wood
  • Topcoats preserve wood. Of course this implies that the wood is kept dry and inside the living portion of the house. Unheated basements and attics are very bad for wooden things. Most unheated basements are very damp, which accelerates wood deterioration.  Attics have the opposite problem in that they are extremely hot dry places. Attics dry out wood, which makes it very brittle. The hot dry air also ruins many wood finishes.

Choosing a Finish

  • There are a huge number of different products to color and protect wooden items. Listed below are just a few of the different materials and processes available. Which one to choose? Start easy and branch out as the urge strikes. Our rubbing finish is rather durable and is very easy to get right. Brushing on finishes is much more tricky and usually will not produce as nice a finish. Here, the more you pay for a brush the better the result will be.

Types of finishes

There are basically three types of finishes

  • Evaporative
  • Reactive
  • Coalescing

Wax is an evaporative finish because it is dissolved in turpentine or petroleum distillates to make the familiar soft paste. After these distillates evaporate all that’s left is the wax.

Reactive finishes use solvents such as white spirits and naphtha. Oil varnishes and linseed oil are reactive finishes which change chemically when they cure, unlike evaporative finishes. At cure, the solvent/thinner evaporates and the resins cluster tighter together, and then a chemical reaction occurs causing the resins to cross link in a different chemical format—like loose scaffolding that suddenly bolt together. Scuff sanding is necessary between layers of cured finish so that subsequent applied layers have something to grip onto effectively. The solvent won’t re-dissolve the cured film, e.g., white spirits does not soften cured oil based varnish.

Note that pre-catalysed and post catalysed “lacquers” (aka acid catalysed lacquers) are reactive finishes. The term lacquer is, in this sense, used inconsistently from product to product.

The oil based varnishes dry from the top down by reacting with oxygen. The catalysed lacquers dry from the bottom up (which is like the evaporative finishes) and the solvents migrate upwards to the film surface and then out leaving behind molecules that then crosslink.

Tung oil and linseed oil are reactive finishes that cure by reacting with oxygen, but do not really form film finishes when cured.

Water based finishes generally fall into the coalescing category.

Oil Recipe used in Collinsville Wood Shop:

We use a half and half recipe of boiled linseed oil and oil based polyurethane. The linseed oil allows for the finish to become a Rubbing Finish, in that the excess finish is wiped off almost immediately after it is applied. This finish does not produce a fast shiny finish, but requires at least 3 coats separated by at least 24 hours between coats. Sometimes even more is needed, but this finish is somewhat fool-proof, and will produce a smooth even topcoat.

Comparison of different clear finishes

Clear finishes are intended to make wood look good and meet the demands to be placed on the finish. Choosing a clear finish for wood involves trade-offs between appearance, protection, durability, safety, requirements for cleaning, and ease of application. The following table compares the characteristics of different clear finishes. ‘Rubbing qualities’ indicates the ease with which a finish can be manipulated to deliver the finish desired. Shellac should be considered in two different ways. It is used as a finish and as a way to manipulate the wood’s ability to absorb other finishes by thinning it with alcohol.  The alcohol evaporates almost immediately to yield a finish that is completely safe but shellac will attach itself to virtually any surface, even glass, and virtually any other finish can be used over it.

Appearance

Protection

Durability

Safety

Ease of Application

Reversibility

Rubbing Qualities

Wax

Creates shine

Short Term

Needs frequent reapplication

Safe when solvents in paste wax evaporate

easy, needs sanding

Can easily be removed with solvents

Needs to be buffed

Shellac

Some yellow or orange tint, depending on grade used

Fair against water, good on solvents except alcohol

Durable

Safe when solvent evaporates, used as food and pill coating

French polishing difficult technique to master.

Completely reversible using alcohol

Excellent

Nitrocellulose lacquer

Transparent, good gloss

Decent protection

Soft and somewhat durable

Used toxic solvents Good protection is needed, especially if painted

Requires nice equipment. Kick-on products also available

Completely irreversible

Excellent soft finish

Conversion varnish

Transparent, good gloss

Excellent protection against many substances

Hard and durable

Uses toxic solvents, including toluene. Breathing protection is needed

Requires spray equipment. Used in professional shops only

Difficult to reverse

Excellent hard finish

Linseed oil

Yellow warm glow, pops grain1, darkens with age

Very little

Fairly durable, depending on number of coats

Relatively safe, metallic driers are poisonous

Easy, apply with rags and wipe off. Takes relatively long time to dry

Needs sanding out as oil is absorbed

None

Tung oil

Warm glow, pops grain1, lighter than linseed

Water resistant

Fairly durable, depending on number of coats

Relatively safe, metallic driers are poisonous

Easy, apply with rags and wipe off. Faster to dry than linseed oil

Needs sanding out as oil is absorbed

None

Alkyd varnish

Not as transparent as lacquer, yellowish/orange tint

Good protection

Durable

Relatively safe, uses petroleum based solvents

Brush or spray. Brushing needs good technique to avoid bubbles & streaks

Can be stripped using paint removers

Fair

Polyurethane varnish

Transparent, many coats can look like plastic

Excellent protection against many substances, tough finish

Durable after approx. 30 day curing period

Relatively safe, uses petroleum based solvents

Application requires some level of skill

Can be stripped using paint removers

Bad, coats do not meld leading to white rings if rubbing out cuts through coat

Water-based polyurethane

Transparent

Good protection. Newer products (2009) also UV stable

Durable after approx. 10 day curing period

Safer than oil-based, fewervolatile organic compounds(VOCs)

Brush or spray. Fast drying demands care in application techniques

Can be stripped using paint removers

Bad, coats do not meld leading to white rings if rubbing out cuts through coat

Oil-varnish mixes

Similar to oils unless many coats applied, then takes on characteristics of varnishes

Low, but more than pure oil finishes

Fairly durable, depending on number of coats (archaic product, little used with the availability of modern finishes)

Relatively safe, uses petroleum based solvents

Easy, apply with rags and wipe off. Faster to dry than linseed oil

Needs sanding out as oil is absorbed

None unless many coats applied