Finish is Scratching

[Steve Aiken]

I put 3 coats of water-based polyurethane over top of an oil-based stain on white Oak. The finish is very easily scratched with the fingernails. I llet the stain dry for a full day before applying the poly. And I followed the manufacturers instructions for the poly. Any ideas on what the problem might be?

Steve

[Jim Becker]

I suspect that the oil-based stain was really not fully cured--if there was still any odor, that would be a clue towards that. Did you also lightly scuff between coats of the poly? Polyurethane-based products don't like to stick to themselves, let alone anything else. When you combine that with putting water-bourne over potentially oily surface, something has to give.

[Steve Aiken]

I did scuff between coats of poly. The scratches go right through to the wood. It's a *little* maddening. I guess the only fix is to sand down to bare wood and start over. Would oil-based poly "stick" better/ be more scratch resistant? Or do you think the major problem is the uncured stain?

Steve

[John Hemenway]

I agree w/ Jim. The oil was not cured fully. After the odor goes away from the oil stain, you can seal it with shellac (dewaxed) and then use the poly.

[Steve Aiken]

Would you say there is a significant difference between water-based and oil-based polyurethane in terms of scratch resistance?

Steve

[Jim Becker]

Would you say there is a significant difference between water-based and oil-based polyurethane in terms of scratch resistance?

They are entirely different products. Oil-based polyurethane varnish has the polyurethan resins incorporated into the mix during manufacture and they are transformed via heat into a mixture that will "cross link" when it cures into a durable surface. Because of the polyurethane resins, it's actually a somewhat "soft" surface which in turn, gives it abrasion resistance. The actual difference in this respect between an alkyd or phenolic resin varnish is not as great as the marketers make out, but that is also dependent on the product, itself, and how it's formulated.

Water bourne "polyurethane" is generally an acrylic coating with polyurethane resin added. I'm really not sure if it's as "durable" or "scratch resistant" as the oil based variety; anecdotal comments I've seen over time seem to indicate it's not, but those comments were also likely based upon products easily available to the consumer market. Companies, such as Target Coatings and Fuhr International have been developing some pretty nifty new finishes, including water bourne polyurethane and "conversion varnish" products that do approximate the durability that their oil-based cousins have.

I only have one piece in my house that has polyurethane applied by me...the kitchen table. Oil based...many coats wiped on. It's scratched, but coincidently not by dining but by an HVAC contractor who set his tool belt down with "less than care". The dresser top in the girls' room has a water bourne "poly" (probably Minwax) and looks as good as the day I refinished it many years ago, although it's only endured a month of "kid activity". My kitchen is finished with water bourne acrylic and (underneath the dirt), remains as scratch free as it was when the cabinets were installed in 2003.

Bottom line...water bourne finishes can be very durable. The specific product can affect that property. But I still think your issue is more to the application (uncured oil-based stain) rather than the product, itself...

[Steve Aiken]

Thanks, Jim. Sadly, I will have to try again. I will wait longer between the stain and the poly.

Steve

[Steve Schoene]

Besides waiting for the stain to cure, I concur with John that a DEWAXED shellac "barrier" coat or two may help. It needs to be scratched a bit too for the poly to have something to grab to. Oil based poly seems to adhere a bit better than waterborne--especially waterborne finish available in big boxes. (And, non-poly varnishes with traditional alkyd or phenolic resins are even less picky--though in the end all varnishes need a mechanical bond to really adhere well.)

[Howard Acheson]

Couple of things. Jim is correct, the stain was not fully dry. You must let it dry for 3-5 days if you intend to overcoat an oil stain with a waterborne finish. Many will also overcoat an oil stain with a dewaxed shellac to insure better adhesion.

In addition, poly varnish needs to be fully cured before it develops full adhesion and scratch resistance. Full cure can take 3-4 weeks and care should be taken until that time has passed.

[Jim W. White]

Howard brings up another point. It takes some time for Poly to FULLY harden. I found this especially true of the water based Poly I used. (General Finishes) I would say that the water based poly takes twice as long as the oil based to reach installation-ready hardness. I also concur with the others that it was necesarry to wait several days between popping the grain with the oil based product and proceeding to the water-based poly.

Having said that I now find it to be extremely durable and just as scratch resistant as the oil based poly's I've used in the past.