Appearance Defects with Waterborne Finishes Applied to Plywood

      As plywood quality declines, finishing problems multiply. Here's a look at why plywood is so hard to finish these days. November 29, 2014

This seems to be a recurring problem for us: ripples on the surface of the plywood visible in the reflection of light. In researching this issue, we have learned about the different substrates for the plywood available from our local distributors. There is fir, poplar, and a combination called CFC. The last has a thin layer of MDF on both sides just underneath the show veneer. The CFC core does provide a very smooth surface, but we have an engineering problem with the extra weight on one of our products. Our most popular plywood is a B-2 maple, rotary-cut veneer. We have learned to specify poplar core instead of fir, and we have been getting a pretty smooth surface and good finishing results. When all plywood arrives, it appears to be very smooth. We sand it with 220, per the finishing supplier's recommendations, then add color, if requested, followed by two good coats of pre-cat water-borne lacquer. With the poplar core, all seems to go pretty well.

Our latest issue, this past week, was that we need to use 1/2" plywood instead of the normal 3/4" for a particular engineering reason. The only 1/2" plywood our regular supplier had used a fir core. The results were horrible. The underlying knots and ripples telegraphed through to the final finish. The raw plywood looked quite good, was sanded per our normal procedure, color was added, then the pre-cat lacquer. The imperfections did not appear until the final coats.

Today, in attempt to resolve the issue, we acquired some 1/2" plywood with, we hope (the distributor was not sure) a poplar core, from a different supplier. We will cut out the parts we need, then use some of the scrap to test to see if the imperfections are there before we finish the actual work pieces.

My questions are:

1. Has anyone else experienced this issue? If so, how did you resolve it?

2. Is this something that happens only with water-borne finishing materials? Or, does it happen with solvent-borne as well? Our thoughts are that both water-borne coats are full-coats (no thin, almost dry coat for the first pass), and that the moisture from the pre-cat is penetrating past the show veneer and causing the substrate to swell. Any imperfections in the substrate (knots, uneven veneer-slicing, etc.) will then telegraph through to the top. Sanding before the finishing process will obviously not do any good since the swelling occurs at a later step. Furthermore, sanding after the pre-cat is applied will not work because that will remove the applied color from the high spots. What is going on here?

3. Should we pre-soak, with distilled water, the plywood before we even start the process? We could possibly do this to the entire 4x8 sheet to get the substrate to swell, then let it dry and sand it smooth. Sanding large uneven pieces with a 6" R/O sander does not sound either efficient, or even possible to get it smooth. The finished product has areas that are about 8-10 square feet that are vertical show surfaces. Any light from an outside window or door will reflect on the surface, so this idea does not sound too good either. We need to re-work this part of the project and deliver it early next week. Sounds like some weekend work, but we need to resolve this. Any advice would be appreciated.

Forum Responses
(Finishing Forum)
From contributor G:
Today’s plywood is marginal - plain and simple. With the .017" veneers they have no hiding ability. With the fast as possible processing of the core materials there are overlaps and missing spots. If you want a clean flat surface for paint the only way to go is with a manufactured product instead of a natural product.

From the original questioner
Just to clarify, this is a stain-grade application, not paint-grade.

From contributor J:
Contributor G is right. Like so many things there is a race to the bottom to compete on price. For the plywood mills, the stumpage price is so high the difference or margin is so tight. All the companies cut the quality to both survive and try to stay in the black. Use baltic birch or apple ply.

From contributor T:
If I understand the problem correctly the substrate is telegraphing through the veneer. The main problem is the thickness of the veneer. There’s no way to rectify this without laying up your own ply - not cost effective. One thing you may want to try with your WB coatings is light coats to start. We sprayed primarily WB coatings for ten years and had similar problems with birch and maple veneers. If you’re putting stain down first you should try a dye stain with toner coats to build up your color. Dye stain will not add as much moisture to your substrate. Adding water to the veneer and substrate can and will create the telegraphing of the substrate through the veneer. After stain is applied lay down a very light coat - about half a mil or enough to barely feel the finish on your fingertip.

This coat will dry very fast especially if you have good air flow (fans or forced air) and temps around 70. After coating has flashed off (10-15) mins apply another light coat just slightly more of a mil build. Let this coat flash off then apply a light 2-2.5 mil coat. Now sand coating and apply one or two more 2.5-3 mil coats and sand between these coats if necessary. Solvent products will not raise the veneer or substrate like WB products will. What products are you using? Why can't you sand between coats? Are you burning through the stain or a toner coat? You should be able to add a small amount of tint or stain to your top coat to hide any slight burn-throughs.

From contributor D:
To the original questioner: You said you are using B-2 grade plywood for a stain grade product. That is a B face with a #2 back. I use that product for interior box parts. You need to start with an on-grade panel. The results may surprise you. Yes the panels with MDF under the skins are going to be the smoothest surface.

From contributor P:
MDF core ply would solve the flatness issue, but may not satisfy your structural requirements. There are also hybrid cores, with wood in the center and MDF layers below the surface veneer.

From contributor G:
B2 or A1, you will still have the same marginal cores and same ultra-thin veneers on top of them. A better veneer will not solve any issues of the poor quality plywood they have these days. If you want flat you go with an engineered core, particleboard or MDF.

From contributor M:
I have not found modern A-1 grades of maple to be any better in their core. The veneer may look cleaner but not so much the core. Your safest bet is to get maple laid up onto some 3/4" (or 1/2") MDF, but this isn't cheap. As suggested above, you may find baltic birch to be a reasonable alternative. Usually the core on these is far more predictable and uniform. I have not tried the apple ply mentioned above. If anyone could clarify this, I would appreciate it. Finally, you may have some luck sanding your plywood with a large hand-block (4x10 or so, basically a half-sheet) and 120 grit sandpaper, sanding with the grain direction, to knock off the tops of the ripples. However, you may burn through the veneer, and this is only a band-aid to limp you through a job that's already cut. It's not a solution to future problems.

From Gene Wengert, forum technical advisor:
From a technical point of view, what happens when we cut veneer (and can also happen any time we use too much pressure), is that some of the wood cells are compressed into the surface rather than cut cleanly. When the water hits them, they pop back up (sometimes called springback). I have seen this is plywood for many years and also in solid wood. The reason is that we are using WB finishes. The solution is to use a water pre-finish treatment that will pop the fibers back up and then sand everything smooth.

Note that we also have a phenomena called telegraphing, where something inside the core - compressed wood, uneven surfaces, joints, etc. will telegraph or show up through the face veneer. This reflects some instability in the core and perhaps poor surface preparation before laminating. In especially troublesome cases, a non-water adhesive system can be used for laminating. Caution: Within a short time, all laminating will have to be done using a non-UF adhesive, which means a WB adhesive. We can expect that some people will have trouble due to the water and their inadequate manufacturing practices that a WB system requires.

From Gene Wengert, forum technical advisor:
I meant to include: I have seen excessive pressure from a hand or palm sander create compression that showed up after WB finishing, so we are not talking about a lot of pressure with the moderate hardness species.

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