View Full Version : Question on Shellac and plywood
01-03-2009, 09:20 AM
I know that any staining of hardwood plywoods can result in blotcy spots, especially if their are voids under the venear face.
My question - would a good healthy coat of a sealer such as Zinnser Shellac BEFORE applying any oil or stain help reduce the risk of an unwanted dark spot on the plywood face?
Specifically I am looking at Maple - but this is an "in general" question.
I don't have any to test on right now - and you'd need a fair size piece to be sure. So... I guess this is a "theoretically" type question.
01-03-2009, 10:28 AM
Shellac is VERY commonly used to level the playing field, so to speak, before stain. It should do the same for ply as it does for raw wood.
-- Tim --
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01-03-2009, 10:53 AM
Zac, et al...
On the surface, at least, I might take issue with your choice of the words, "a good healthy coat of a sealer..." Pre-stain sealers should be employed minimally and hence lend nothing visible to the finishing process...So for the sake of discussion, let me use this as a take-off point and perhaps provide you with a loosely applied formula that has worked for me in recent years.
Recent years?...Yes, only because the quality of commercially available A or A-A class veneer plywood has deteriorated to such a level that it's nearly impossible to achieve a uniformly attractive finish without the dreaded, random patchwork that you refer to as dark spots...It seems to matter not who the supplier is: IMHO it's quite pervasive among the several vendors here in Houston.
My remarks presuppose that you have some sort of reliable spray delivery system...With respect to color (or tone), clarity and sheen determine what you want to achieve as a final appearance for your maple (or other species) faced plywood...You can do this on a scrap or cut-off of the plywood in question, experimenting with your dye-stains, pigmented oil stains, top sealers (if any) and finish top-coats.
(1) After suitable surface preparation with abrasive paper routines, apply a single application of 1-pound cut shellac to the surface(s) to be finished...This application should be a just barely wet wet coat and within a reasonable climate (>65F and <60% humidity) should be dry and ready for a scuff sanding with 320 free-cut paper in about 20 minutes or so...The key here is to apply a uniformly wet (just barely wet) coat...No more.
(2) Apply any dye-stain of choice, suitably thinned for spray application...This dye-stain can be thinned with menthanol or lacquer thinner -or- even water but the water option requires more care and longer wait before moving on to the next step...Whatever solvent type you choose, a weaker concentration is better than a stronger concentration...Open the air-control and volume setting of your sprayer head to make a wide fan and generous delivery...What you want to achieve is a wet coat of a minimally concentrated dye-stain...This will insure a uniformly applied tone (or undertone for your pigmented oil stain) and work to minimize any holidays or pass borders...Need more color?...Simply apply more dye-stain with successive passes...This process goes quickly because of the short intervals between applications.
(3) When the dye-stain has dried sufficiently, look at the tone you have achieved over the entire workpiece surface...If you see areas where the density is less than desired, repeat a quick spray application directed at these areas with the spray head fan narrowed and the delivery volume reduced.
(4) After you have achieved the dye-stain tone you want, you can apply the pigmented oil stain...For projects requiring a very light tone and greatest clarity of wood grain, thin the pigmented oil stain with naphtha sufficiently to apply with your spray-gun...As with the dye-stain application, open the fan but keep the volume to a manageable level...Not too much volume otherwise the pigmented oil stain can pool according to your inconsistent spray head movement(s)...What we are attempting to achieve here is a gradual build-up of pigmented oil stain by making several passes with the reduced (thinned down) stain...For areas that need slightly more pigment, repeat the "tone balancing process" as described in the last sentence of paragraph 3 above...You may choose to narrow the fan and reduce the volume for this balancing application...For darker tones requiring more pigment on the surface, apply more material with subequent passes after the previous application has "flashed" to prevent pooling and uneven results...Throughout this procedure, you should stir the contents of your spray-gun every 15 to 20 minutes to insure uniform delivery because the oil stain pigment will tend to settle while the gun is at rest.
(5) Protect the freshly applied oil stain work surface from dust or other airborne contaminents and after a suitable elapesd time (usually 4 to 8 hours) allowing the pigmented oil stain to dry, you can then proceeed with the remainder of finishing steps including sealer, toner(s), shading and topcoats of lacquer, shellac or varnish (preferably all applied via a spray delivery system).
As I recall, I've used this routine with anticipated success on cabinet grade plywood of white birch, Southern maple, cherry, walnut (without grain filler) and teak (without grain filler) as well as commercially manufactured door skins and interior doors of birch...The process described above has been accomplished with the use of typical syphon cup guns at a regulated delivery pressure of approximately 30psi to 32psi for all applications described above.
01-03-2009, 11:28 AM
First off, one needs to understand what causes the blotching of wood, be it ply or solid. Some woods such as cherry and pine have significant differences in grain (I think it is believed that these differences are between the early wood and late wood of the growing season) which leaves some areas of the wood more porous and susceptible to absorbing more of what ever liquid is applied to them. What ever you use as a conditioner needs to be thin enough to be absorbed into these areas where it will harden and seal the grain so that less of the stain or dye is absorbed while being left closer to the surface of the wood in the areas that are less absorbent where most of it is easily sanded away allowing more penetration of color in those areas.
In my opinion, even the Zinnser Sealcoat, a 2# cut of shellac, is too thick a coating to use as a pre-stain conditioner. It was designed as something to completely seal the grain to prevent stains in the surface from migrating through and into what ever top coat is applied over it, and as an intermediate between two non-compatible coatings. As such, I generally cut the Sealcoat 1 to 1 with denatured alcohol before using it as a sealer. It also serves to raise the grain on wood that is prone to that. A soft cotton wrag wrapped around a ball of cheese cloth or some similar medium works well for applying the Sealcoat quickly and evenly. Once it has dried, a light sanding using 320 or 400 grit paper with a sanding block leaves a baby-butt smooth surface ready to accept the next step in the finishing regimen. I use some mineral spirits on the surface when sanding to prevent the paper from loading up.
This should work with your maple ply. As to the voids, you might try to locate a supplier in your neck of the woods who carries ply with MDF plies beneath the outer veneers. This stuff is very stable and void free and remains exceptionally flat.
01-03-2009, 12:39 PM
I hope Zac got as much out the replies as I did.
Since I'm attending a seminar on "Finishes" in about an hour at a new Woodcraft store, Bruce and Jerry's responses add good background info before I go. Thx, guys.
I can now ask "intelligent" questions (instead of my normal ones. :-)
01-03-2009, 02:34 PM
I appreciate the very detailed response. A couple things.
1) I guess it was just worder poorly. "A good healthy coat"... I realize that the more I put on - the more it literally seals the wood, and the more it effects the finishing process. I didn't mather lather it on heavy - it was just worded poorly.
2) Your method sounds great..... for spray guns. This would be a complete "hands only" finish. Now - much of what you said can be transferred to using a "by hand" method - but stressed heavily, the use of a spray gun. Nope - don't have one.
01-03-2009, 02:40 PM
Oh, yes Jerry. I know that the heavy shellac cuts will go on like syrup, effect the finish, AND nearly completely seal off the wood pores.
AND - I recently learned that Zinsser Shellac from the can comes as a 2 pound cut...I never realized that before. So, YES - I had planned to cut it by 1/2 with Denatured Alcohol.
I guess to be very specific with my question - I meant.
"Would a 1 pound cut of shellac applied thinnly to maple ply help cure some of the possible blotchyness that is often associate with maple"
I was aware (somewhat) of what causes the blotchyness in wood... BUT - wasn't sure if the same train of thought to prevent it, carried over into very thin plys.
As far as MDF plywoods.... YES - I have read some fantastic things about them - and I will be looking for an MDF core if possible. One problem.... I don't know of a single local supplier for plywood other than Big Box stores. There is one cabinet shop/custom trim shop in my area... not a huge operation - but one that is operating. I don't know any of the guys, but maybe they'd be able to tell me where a place to look would be.
01-03-2009, 06:26 PM
I sure don't want to make this more confusing, but it would be very easy as you can see. The principle is basic. The first consideration is the specie of the wood or plywood and what your intended finish schedule will be. The surface preparation may dictate how much stain will be absorbed and/or, its uniformity. Pre-stain/dye prep could solve some of the problems. Some species don't benefit from being sanded too smoothly.
Whatever pre-stain conditioner that is used, it can be applied in a state of concentration that prevents even coloring by sealing too much. Doing samples is the best way to "get the hang" of what to use and how to apply.