Whether teak, mahogany, oak or douglas fir, Natural stained or varnished wood sills are very pretty.
But if not maintained, they can get pretty raggedy looking and even rot if water is allowed into unsealed surfaces.
Prepping a wood sill
If finish is failed and wood is very discolored it is often best to strip the sill. This can be done chemically or with a power sander.
After stripping and or sanding, often the wood has deep discolorations that can be removed with a specialized oxygen based (oxalic acid) wood bleach. Do not use household bleach (sodium hypochlorite) as this will damage the lignin in the wood. Sometimes it takes a few treatments of bleach to get back to like new looking wood
At this point you have a choice to go with a penetrating oil, straight clear finish or to stain and then finish. Staining is helpful when you want to blend the sill into the look of the adjacent door (click for more on wood doors)
Choosing your finish
Penetrating type oils:
Teak oil, Sikkens SRD, TWP: these are easiest to apply and can give a color to the wood of your choosing. They are super easy to maintain but have to be recoated more often (every six-twelve months)
Varnish gives the classic look that, with 3-4 (or more!) coats, can give a smooth glass like finish. This finish can last a long time if not subject to direct sun. Drawbacks are that you must sand between coats and if you let it go too long and some areas fail, the exposed wood will discolor and you will have to strip the sill and start from scratch.
Tinted film forming stain/finish systems:
I believe these are the best balance of durability and ease of maintenance. I use these on garage doors and front doors as well (Click for more info). Products of this type are:
Sikkens Cetol Marine is a film forming translucent clear tinted finish that looks great on teak and mahogany as it has an amber tint that accentuates these woods. Cetol Marine also comes in a teak color that is suitable for teak, mahogany AND oak. Two coats minimum on bare wood with 3 being just right. It dries to a satin sheen and does not need sanding between coats. When maintenance is required, just recoat and failed areas will blend in as this is a tinted product
Sikkens Cetol 123 Plus: this an oil based system where the Cetol 1 is a stain available in various pre-packaged colors; it is applied to soak in to the wood and give a deep rich color. Cetol 23Plus (meaning 2nd, 3rd coats or more) is a film forming satin finish much like Cetol Marine but tinted in more colors and can be used on any type of wood
Water borne acrylic systems usually have a stain product and a clear finish product that can be tinted with the stain to “shade in” damaged areas. These can be very durable and dry superfast. Fast drying can really help in productivity but it takes a real pro to use these or finish can look uneven Products of this type are
ECS (Environmental Coating Systems) brand: manufactured in San Diego, I use this both inside and outside. For outside applications I use the “UV Extreme” and have finished beautiful mahogany doors
Sansin brand: is a stain/clear coating system out of Canada where natural wood finishes are the norm. We used this finish on a seaside project with Italian Albertini mahogany windows. (Click for more on Sansin)
General Finishes brand: this is a favorite of many wood workers and painters and is probably the most available of as it is distributed across the USA.