One thing to keep in mind- varnish and poly harden on the surface of the wood. I don't relish the idea of eating plastic in my salad. They both will chip if you use 'em on cutting boards….maybe it'll add that extra flavor that will make people ask you- "What's the secret ingredient, Steve?"(LOL) I think Marc is basically making a thin wiping varnish. I prefer to use mineral oil or coconut oil as a wipe-on for my cutting boards. Both are food safe and don't go rancid…..........Wes
"Polyurethane" can be a varnish… ...or it can be lacquer, or water-base…
"Varnish" usually simply means it's some sort of cross-linking oil-based clear film finish. Cross-linking means that it chemically cures, vs. drying due to solvent evaporation. The big cue is what happens if you wipe the dry film with it's own solvent. Varnishes wiped with mineral spirits suffer no damage. Shellac and nitrocellulose lacquer, which are both evaporative finishes, will dissolve over and over again when wiped with the correct thinner.
The words polyurethane or urethane refer to the resin suspended in the clear varnish binder. There are also phenolic varnishes, like Pratt and Lambert 38, that use a phenolic resin in place of urethane. Other varnishes may include natural oils that polymerize as they combine with air, solidifying the resins.
Most varnishes are non-toxic when fully cured, but those claimed as "food grade" have undergone more testing, and have a legal department to answer questions. Think of it this way… Writing "food grade" on the can means a certain small percentage of sales will be added for food contact uses, but the manufacturer may be open to new legal actions based on the label. Not writing "food grade" on a label means you aren't guaranteeing anything, even if the product is known to you to be non-toxic when cured.
Just a slight difference with what Barry said, P&L 38 isn't a phenolic varnish, it's actually an alkyd resin/soya oil formula. Waterlox original is a phenolic varnish, the only one I know of.
To make varnish you cook resins with a drying oil. The reins are usually a urethane, or an alkyd, or a phenolic; though there are others. The drying oil is typically linseed oil, but can be soya oil, or tung oil; again there are others less common. There are numerous other finishes labeled "polyurethane", the manufacturers believe that magic word will sell more stuff, even if it's wrong. Water borne finishes, for example, are mostly an acrylic resin coalescing finish, yet add a very small amount of urethane and put that on the label, viola! you have a winner (at least in theri deceitful minds). As a rule, if someones refer to "poly", I'm assuming they are referencing the oil based varnish…something that is getting more and more less likely.
1. No difference; they're meaningless marketing terms for a film finish.
2. Any commercially available film finish is "food-safe" once cured. The same goes for BLO and tung oil.
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