by Rick Hill
Q.
We are a manufacturer of wood office furniture, and we have recently experienced a green blush that shows up after our final coat of pre-catalyzed lacquer. Our finishing steps are as follows:
1. Spray NGR (Non-Grain Raising)
2. Wipe on oil stain
3. Tone with a light coat of NGR
4. Spray sealer (pre-cat)
5. Spray topcoat (pre-cat)
We follow the usual sanding and drying steps.
A.
You may have two unrelated causes contributing to the green blushing:
Cause No.1: Blushing is usually caused by trapped solvent or trapped moisture under a topcoat.
Cause No.2: A color shift to green in an NGR aniline dye stain (if that is what you are using to tone) is often caused by a reaction between the acid catalyst in the lacquer and the aniline dye.
Without knowing the color of the stain, brand or dry times I can only offer you possible solutions.
You can correct No.2 by adding a non-catalyzed sealer over the dye stain. To promote adhesion I would use a Modified vinyl adhesive sealer over the dye, then seal with my normal precat sealer.
No.1 requires a little more explanation. Oil base stains are notorious for causing blushing under catalyzed finishes. If we think about the nature of oils, we realize that they take forever to dry. Depending on the type of oils used in your stain, they may feel dry to the touch and yet are still SLOWLY evaporating.
Many of the retail stains like Minwax use linseed oil and mineral spirits. These are fine for homeowner use under uncatalyzed coatings. Uncatalyzed coatings allow the oils to permeate through the topcoat as they dry, or they can be wiped down with solvent to open up the coating and allow the oils to escape. Catalyzed finishes are resistant to oils and cannot be opened by a solvent wipe so the oil is trapped under the topcoat and causes the blushing or cloudy effect. Check with your supplier or MSDS to see if mineral spirits or other slow drying oils are in your stain.
Blushing is also caused by moisture trapped under the coating. To determine whether trapped solvent or trapped moisture is cuasing the problem, coat half of a sanded board with stain and leave half with no stain. Spray your sealer and topcoat. If the blushing is on the unstained side, check your moisture content. It should be no more than 6-8%.
Also, check the dry times of your system. If the sealer is not completely curing before spraying your topcoat, its inherent solvents could be trapped under the topcoat.
My advice is to switch to a fast dry solvent or water base stain that will not have all the inherent problems that oil has. By continuing to use NGR Dye stains in combination with the stains, you should achieve the same look without the slow dry time.
Rick Hill is an independent representative and consultant for industrial wood finishes. He has been involved in the woodworking industry for 12 years, and has been known to actually hold, shoot, and clean a spray gun.
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