What causes this?
- Thread starter dmraco
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Keyser Soze
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One of the common terms for that appearance is cratering. It is caused by a physical incompatibility between something in/on the wood and the solvent/base of the finish.
Essentially, the solvent is repelled (e.g. oil v.s. water) by something in/on the wood, so the still wet layer it crawls/migrates away from wahetever it does not like - leaving a depression in the film (or even an actual bare spot.) If you keep applying layers of finish the problem usually perpetuates leaving even bigger craters. the only reliable solution is to strip and seal the offending area.
Silicone is a very common culprit and very hard to get rid of, but some oily woods will cause the problem all by themselves.
This is exactly the reason most wood finishers use some kind of seal coat before applying a topcoat.
Essentially, the solvent is repelled (e.g. oil v.s. water) by something in/on the wood, so the still wet layer it crawls/migrates away from wahetever it does not like - leaving a depression in the film (or even an actual bare spot.) If you keep applying layers of finish the problem usually perpetuates leaving even bigger craters. the only reliable solution is to strip and seal the offending area.
Silicone is a very common culprit and very hard to get rid of, but some oily woods will cause the problem all by themselves.
This is exactly the reason most wood finishers use some kind of seal coat before applying a topcoat.
Keyser Soze
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If it is 'in' the wood it often can't even be removed by mechanical means (sanding) because it just keeps being transferred back into the surface. Again, that's why sealers are used - they from a thin film over the offending substance(s) preventing contact between the topcoat and the offending contaminant.
Shellac is, for most situations, the all purpose sealer - it sticks to just about everything and just about everything sticks to it. Bullseye even guarantees their sealer under 'all' types of finishes right on the front of the label.
Shellac is, for most situations, the all purpose sealer - it sticks to just about everything and just about everything sticks to it. Bullseye even guarantees their sealer under 'all' types of finishes right on the front of the label.
Patrick from Davis
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Just to note, silicon oil causes all kinds of nightmares when you try to get rid of it. You wipe and wipe and wipe with naphtha and still things peel up.
Patrick
Patrick
Keyser Soze
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You can (and probably should) seal before applying any final finish. It is best to check compatibility (either via the label, contacting the manufacturer, or doing a test piece) before proceeding.
Sometimes it's not absolutely necessary (e.g. putting an oil based poly over boiled linseed oil, or an oil based stain.) But any time you are courting compatibility issues (say putting a water based poly over an oil stain) it's a good idea.
It is likely essential anytime you are doing a refinish, especially if you do not know - for sure - what has been applied. As others have mentioned silicone is an insidious substance. It will penetrate many finishes and get onto the wood - once it is there it is all but impossible to remove. Flame scorching (aka burning) the surface then sanding being about the only foolproof method.
Sometimes it's not absolutely necessary (e.g. putting an oil based poly over boiled linseed oil, or an oil based stain.) But any time you are courting compatibility issues (say putting a water based poly over an oil stain) it's a good idea.
It is likely essential anytime you are doing a refinish, especially if you do not know - for sure - what has been applied. As others have mentioned silicone is an insidious substance. It will penetrate many finishes and get onto the wood - once it is there it is all but impossible to remove. Flame scorching (aka burning) the surface then sanding being about the only foolproof method.
