Water-Based Polyurethane Over Boiled Linseed Oil

Water-based poly isn't compatible with BLO. Now this furniture maker has to strip off the poly and apply a more suitable top coat. October 1, 2010

I built a dining table out of solid walnut and wenge. I finished it with boiled linseed oil and then put a first coat of water-based polyurethane on it. I'm not sure if I am seeing it right, but there seem to be areas actively developing under the poly coat that look like the linseed was never applied, and it's just the raw walnut with a poly coat over it. Should I not have finished the table with this combination?

Forum Responses
(Furniture Making Forum)
From contributor C:
Those two compounds are not compatible. Water based poly does not require a sealer. Also, linseed oil takes a very long time to dry sufficiently for top coating. Most likely it was not fully cured.

It sounds like you built a beautiful table. If you are not a finisher, I would suggest having it professionally done. A great piece of furniture can quickly be ruined by a poor finishing job.

Having said that, if you have the equipment and skill, use a pre-cat lacquer. It is durable, water white and much easier to apply, maintain and repair. Also, strip that water based poly off and stay away from linseed oil unless you are going to be using an oil-based finish.

From the original questioner:
Thank you. Is it possible to reverse the application of the linseed? I am guessing not since it soaks in. I want to pull out the grain and enrich the natural color, but also have a durable clear finish. Would the pre-cat lacquer do all this, or could you recommend a combination of products that would produce this result? If I'm stuck with the linseed, you would recommend an oil-based poly at this point, or are you saying the pre-cat lacquer will work over the linseed? (I am a professional designer getting into furniture design - I am not a professional craftsman or finisher, but until I get more established, I cannot afford to hire people to do that work.)

From contributor B:

Get that water based poly off. Strip, neutralize, let dry. You are correct that the oil will remain. That's fine - just let it dry to the touch, then apply oil based varnish. This can be rubbed on with a rag and burnished as it dries. With this method you can apply as many coats as you like. Use light, thin coats, and allow it to dry between coats.

From contributor C:
The oil will remain in the wood. You can use a straight varnish, but it requires practice and a day between coats. Also, you will need to rub out the finish after you have applied your coats, which is another skill set entirely. If you do opt for a varnish, make sure it is an alkalyd-resin interior varnish, not a spar or marine varnish. They are formulated for completely different environments. Your best bet as a novice is to use a Danish oil (made by Watco). It is very easy to apply and maintain and does offer a small amount of protection.

If I were you though, I would offer the client the option of a professional high build finish (5-7 mil), at cost. If there is any surface that can benefit from this type of finish, it is a dining room table top.

Also, when stripping that water based poly off, use a methelyne-chloride based stripper. Do not rinse with water. Scrub and rinse clean with acetone.

Whatever you decide to do, do it on a test piece first! This may save you additional headaches that at this point, you probably don't need.

From contributor R:
A common finish for cherry is to apply a coat of BLO and let dry for 3 to 4 days, then apply a coat of garnet shellac. After that you can leave as is or apply most any top coat. The point is shellac will stick to almost anything and most anything will stick to shellac. Had you put a coat of shellac over the BLO, you would not have had a problem.

From contributor C:
Shellac is an excellent barrier coat and will dry and adhere well to oil. Two things: traditional shellac is not compatible with pre-cat lacquer. The adhesion is very poor with a pre-cat - you have to use clarified or wax-free shellac. The other thing is that shellac is a very poor topcoat. It is soft and has very little scuff resistance. Also, it comes in only one gloss level - high. The final thing worth noting about either form of shellac is that it cannot take heat. It will blister in prolonged, direct sunlight; even if topcoated, it will break the topcoat film. There is no easy fix for that.

Since what you're finishing is a dining room table, I would not use a shellac based product. You might be able to get by with a dust coat just to seal in the oil.

You say the table is done in walnut. Walnut has open pores that need to be filled in some way. If you have not done that, your table will have a very grainy, unfinished appearance no matter the finish you use.

From contributor L:
At this point I think you have to strip the poly. The oil will have soaked in and you can't really get rid of it. It takes a long time for boiled linseed to polymerize dry. There is a durable slick finish that can be done after the oil has had time to dry. First, use a tinted grain filler that is compatible with oil based finish. Then mix about 1/3 of each: gloss oil based urethane, Thomson's Danish oil (or Watco), and naphtha. Depending on the temperature and the like, you may want to vary the mixture slightly. Next comes the excessive labor part. Apply a saturating coat that you maintain wet for long enough to fully soak in (an hour?), and rub dry with a soft cloth. While this coat is still soaking in you can take wet or dry 320 sandpaper and wet sand the top. This will create a slurry that will be worked into the last of the small pores. Let this dry overnight and repeat the next day. It won't take as long the second day as the surface will be mostly sealed. Let dry and each day for the next week apply a coat of the finish without the wet sanding, using the French polish method (cheesecloth and a fill of cotton rag), each time rubbing the finish until it is basically dry. Caution is now required to never lift the rag as the finish approaches dry or you will tear out the surface. End each sweep by going off an edge. The last coat can be left at the natural gloss level or reduced with one of the rubbing compounds to whatever gloss you like. Don't rub to kill gloss until it is fully dry (several days or more than a week).

Lots of labor, beautiful, durable, repairable finish.