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How to make Wipe-On Polyurethane?
Response To:
Wipe-On Polyurethane? ()

Bill Tindall, E.Tn.
I doubt the wipe-on varnish one buys is simply diluted full strength product for a number of reasons. I don't know if diluting full strength varnish will achieve the same properties as wipe-on Minwax, for example, but I doubt it. If someone experienced in finishing says the result is the same I'll stand corrected.

What properties must it have? The wipe-on viscosity needs to remain low for more time than regular varnish so the finish can be spread and leveled. After application one wants regular varnish to build viscosity quickly from solvent evaporation so sags and runs are avoided when brushing relatively thick coats of varnish. And, the product must pass EPA requirements. Companies get regular varnish to pass by increasing % solids, which increases viscosity. If regular varnish could be sold more dilute it likely would be so it would level easier when brushing. (I always dilute regular varnish now days, where in 1980 I didn't need to )

So, how would a company that understood the chemicals that go into making varnish go about making a varnish that had a long working time, where sags and runs were not a risk? A "long working time" means the viscosity of the varnish stays low for enough time to spread and level the material by "wiping" thin coats.

Viscosity depends on the % solids and the molecular weight of the resin component. Some claim that wipe-on is just diluted varnish. Indeed the initial viscosity of diluted varnish could be made low. But in a thin applied film the solvent will evaporate quickly and the viscosity will increase and make spreading and leveling difficult by wiping.

Oil could be added, the infamous 1:1:1 varnish, oil, solvent recipe. This material would wipe well as the added oil would dilute the resin and keep the viscosity low even after solvent evaporation. But the addition of the oil will result in a soft finish.

A smarter way to make wipe-on is to formulate with a lower molecular weight resin than regular varnish. Upon solvent evaporation the viscosity of the remaining film will remain low and it will still spread and level. The resin remaining after solvent evaporation will become hard and durable upon curing by air oxidation. My bet is that wipe-on varnish is made this way.

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