The light and dark areas are normal for this aged and used wood. Are the darker areas where foot traffic would be concentrated? If so, then I would venture a guess that it is what I call bruising of the cellular structure of the wood. Repeated footfalls break the wood cells and any resin in those cells 'leak out' and penetrate to the wood fibers adjacent and below. We also see this 'damage' to floors that have had heavy objects rolled across them, such as safes or pianos; you can sand the floor absolutely flat, but the roll marks reappear when finish is applied to the floor.

It really is 'character'...but it is sometimes referred to as 'damage'.

The wood will darken when finish is applied; more so with oil based finishes.

For the lightest effect, use a waterborne sealer and then coat with BonaKemi Traffic or another commercial waterborne finish. These finishes are not available to you at your home center.

You can stain natural or golden for a darker effect and then apply a waterborne finish. Fir and other soft woods will 'splotch' when stained, so you have to be careful what stain you use. The best I have used is another product from BonaKemi called DriFast stain.

There are other products that other professionals would use. I have not used those, so maybe someone who has will post with their recommendations.

Good luck.

GlennSFC -- thanks so much for the thorough response ... the "mottling" is indeed in the center areas of the treads (if I stand at the top of the stairs and look down them, I can almost see footprints!!). They are completely and evenly light (no mottling -- looks like brand new wood) at the edges. I'm glad to know that's what it's from (I'm happy to have character, less happy to have damage or remnants of stain...). Sounds as if our best bet for the light effect we want is just a waterborne sealer. I'll ask my contractor (we are remodeling an adjacent area) about the commercial waterborn finish -- I presume he could get it? Thank you so much for the advice!

You've probably had the work done by now. If not, feel free to email me about it.

Commercial quality waterborne wood floor finishes are available from distributors and dealers who sell to professional wood floor finishers. The catalysts are dangerous substances, so be careful how those are handled.

The websites for BonaKemi and Basic Coatings will give you an opportunity to locate a source in your area. Hoboken Wood Floors is just one distributor close to you.