Michael Dresdner » seal

Play musty for me

Michael — Tue, 30 Nov 2010 14:00:27 +0000

Q: My old desk has a kind of musty smell. Can this be improved by treating the wood?
A: Musty smells can come from mildew or accumulations of dust and dirt. Clean the desk, including the interior areas, and kill any mildew with laundry bleach. If you want, you can seal the exposed wood areas, such as the unfinished drawer sides and cabinet interiors, with a coat of shellac, and that will nicely block any smells.

Formaldehyde

Michael — Mon, 29 Nov 2010 14:00:10 +0000

Q: I purchased a 70s-era desk with an oak veneer. Much of the desk is constructed of heavy fiberboard. I’m worried about the formaldehyde and other chemicals off gassing. Is there a way to seal the exposed fiberboard?
A: If it is from the 70s, anything that was going to off-gas has long ago done so, but to answer your question, any film forming finish will seal in formaldehyde. That includes shellac, lacquer, oil varnish, and both oil and waterbased polyurethane.

Crisp masking lines

Michael — Tue, 02 Nov 2010 14:00:56 +0000

Q: How do I achieve a nice crisp symmetrical line when using two different stains? For example I want to stain a light triangle inside a dark rectangle. The stain has bled under the painter’s tape in my tests.
A: There are a variety of techniques, but they depend on both the type of stain you are using and the type of wood. Without knowing both, I can’t advise you of the best course of action. Here’s just one of the many ways to approach this that works well with fairly absorbent wood and pigmented stain.
Stain the lighter color over the entire surface. Mask off what will be darker, and seal the light triangle with a sealer that will not dissolve with the solvent in your darker stain. Now reverse the process; mask off the light triangle using either lacquer tape (typically green) or pin striping tape, (typically olive). Now add the secondary dark stain to the exposed areas. The combination of the correct tape atop a sealed surface should prevent the dark stain from creeping onto the light area, and still give the look of stained wood rather than a tinted finish.

Inked salad bowls

Michael — Fri, 29 Oct 2010 14:00:31 +0000

Q: I want to color walnut sapwood on salad bowls with India ink, then seal it with shellac and salad bowl finish. Would this be safe for food?
A: Yep. By the way, I would let the ink dry then go straight to salad bowl finish. In this case, adding shellac would weaken the finish.

Auntie Oxidant

Michael — Fri, 24 Sep 2010 14:00:07 +0000

Q: I made some replacement handgun grips of cocobolo. Realizing that cocobolo has a lot of natural oils in it I first sealed it with shellac, then filled the pores with a product called z-poxy. I then sanded and applied one coat of Minwax polyurethane. Three weeks later I can make a thumb print in the finish. I think I sanded through my seal coat, because the poly is not curing. Can I remove the uncured poly with mineral spirits?
A: I doubt it, unless it is still fully liquid. What typically happens is that exposure to the raw cocobolo (where you sanded through) will inhibit the cure of an oil based finish, but not keep it entirely liquid. Thus, it stays tacky or gummy.
You’re certainly welcome to try scrubbing off the finish with mineral spirits on a coarse nylon abrasive pad, but in most cases you will have to resort to at least refinisher if not stripper. Since the parts are so small that should not be a big deal. You can also sand back to raw wood, again, since the parts are so small.
For the record, the reason oil based finishes don’t cure over dalbergias, such as cocobolo, has nothing to do with the wood appearing oily. In fact, it contains resins, but not, strictly speaking, oils. The problem is that the wood also contains a natural anti-oxidant.
Think of how citric acid, another natural plant anti-oxidant, slows the oxidation and browning of freshly cut apples if they are dipped in lemon juice. The anti-oxidant in dalbergia woods does the same thing.
Oil based polyurethane and varnish cure by oxidation polymerization; they take oxygen from the air and use it to link up small molecules to make large ones, thus turning a liquid finish into a solid film through chemical linking. The anti-oxidant is the culprit in delaying or preventing that chemical reaction.

Compatibility

Michael — Thu, 02 Sep 2010 14:00:15 +0000

Q: A friend told me that he used nitrocellulose sanding sealer on wood, and then finished it over with an automotive two part clear coat. Does this seem possible? To me it just sounds like the two different systems will not go together.
A: You are correct in worrying about incompatible systems, but as luck would have it, the two your friend used, while not ideal, are compatible. He’ll be fine as long as he did not pile on too much soft sanding sealer below the harder topcoat.

For once, no primer

Michael — Tue, 17 Aug 2010 14:00:09 +0000

Q: We’d like to coat our stripped metal furniture with hammer finish from Rust-Oleum. The local paint person stated this can be applied directly to the metal, but perhaps I am just being old school so I am questioning whether priming first would be better.
A: Rustoleum’s hammer finish is in fact fine to use without any primer, though on an uneven surface, it might make a slightly different pattern than you get on a smooth one. That’s really the only reason I could think of for bothering to prime; to improve the surface of the substrate. If that’s not necessary, don’t bother with primer in this case.

To seal or not to seal

Michael — Wed, 04 Aug 2010 14:00:29 +0000

Q: Should I apply a sanding sealer after staining maple? I read that you cannot use sanding sealer under a sprayed precatalyzed spray lacquer which is what I was thinking of finishing with.
A: That’s half true; stearated sealers, which are one type of sealer, are not compatible under most catalyzed finishes. You must have a positive reason for every step in the finishing process. Sanding sealer is typically used on highly absorbent woods. Maple is not one of them and does not ever require sanding sealer. There are other types of sealers used for other reasons, including some that act as barrier coats allowing finishes to coexist over otherwise incompatible or bleeding stains, but that is not an issue here.

Punky oak

Michael — Mon, 19 Jul 2010 17:11:30 +0000

Q: I am making French doors out of red oak. I got to the sanding stage and I noticed pithy type areas, pithy veins. I sanded them but I was wondering if you had run into this and if so how you handled it?
A: Certain types of red oak, including one we used to call bog or swamp oak, is prone to soft, fibrous, spongy areas. If they are bad enough that simply sealing them and sanding is not enough to firm them up, you can treat them the same way you tread spalted or rotted wood by potting the wood with a wood rot treatment. These come in two categories; thin, usually acrylic solutions that get absorbed and harden within the wood, or epoxy based thicker solutions for truly rotted wood. My guess is that the former will be more than adequate. You can find it at most stores, including the home stores (Lowes, Home Depot), at least in my area. Usually it is in with finishing products and adhesives.

Misguided advice?

Michael — Mon, 12 Jul 2010 18:10:13 +0000

Q: I am finishing some beech and used Zinsser SealCoat in a 50-50 mix with denatured alcohol. After light sanding I applied tung oil but the result was some serious blotching. I’m planning on using shellac as a finish.
A: Zinsser SealCoat is pure, dewaxed shellac, already reduced to exactly the right consistency. That implies two things. First, there is absolutely no reason to thin it when using it as a sealer coat. In fact, thinning it that much turns it from an almost perfect sealer into something almost completely useless. Second, all you need do for a shellac finish is continue to apply coats of SealCoat until you are happy with the results. It’s a very simple and very effective finish. I don’t know what possessed you to put tung oil on wood you want to finish in shellac – perhaps some misguided internet chatter – but as you now know, doing that was both pointless and problematic.