I am using Interlux Schooner to refinish a cedar strip canoe. Is it possible to skip using a red 3M pad between coats? I am curious whether there is a window of time when a chemical bond occurs eliminating the need to sand?
Schooner is a solvent-based varnish. There is no chemical bond. The whole chemical vs mechanical bond thing only applies to cross-linking chemistries that also adhere to surfaces other than themselves, like epoxies and top coats.
The way solvent-based chemistries work is that there are solid components dissolved in a usually petroleum-based solvent (the kind that smell bad). When you apply a coat, the solvent starts evaporating from the outside, leaving a film of solids. Eventually all the solvent goes into the air leaving only the solid components as the varnish finish. (That’s why it’s critical to use thin coats. With a thick coat the outermost layer of the coat will dry and form a solid barrier to the evaporation of the solvent from the inside of the coat. You end up with a soft mushy coat that takes forever to dry.)
So when the first coat is dry and you put on a second coat, the solvent from the second coat dissolves the very topmost layer of the first coat and the 2 coats blend together. When the solvent evaporates, you end up with the second coat dried onto and merged into the first coat, without any chemical bonding.
There are 2 reasons for the sanding. The first is to get rid of brush marks. The second is to make the outer layer of the first coat dissolve more easily when the second coat is applied. The abrasive pad/sandpaper/etc. scratches up the surface of the previous coat so that instead of a large single surface it becomes a collection of microscopic hills and valleys. There are now many more and much smaller surfaces to interact with the solvent from the second coat so they dissolve faster and the second coat has a chance to merge before the solvent evaporates. It’s exactly the same as when crushed ice dissolves faster in a drink than a single big chunk of ice.
That’s why you sand varnish between coats.
Laszlo
Lazlo, Thank you. The best explanation of what sanding accomplishes I have ever read. David
But your superb explanation got me thinking about the application directions for Interlux Toplac Plus which is a) solvent based and b) does not require sanding between coats. Any idea why that is the case?
I remember using the Scotchbrite pad between coats of varnish but I don’t recall if I wet sanded it at coat 4 like I did with the painted hull (Toplac). Is it a good or bad idea to wet sand varnish?
David,
Schooner is an old-style (classic) varnish with an approximately 50-50 mix of phenolic resins dissolved in naptha. Toplac Plus is 50% silicone alkyd resins with added pigments plus less than 13% naptha.
I know that alkyd resins dry slower and harder than plain solvents like naptha or alcohols so that may make the timing for dissolving into the old layer less critical such that it doesn’t need the texturing to help it dissolve faster.
I think that the silicone component helps with the self-leveling to the point that brush marks are not such an issue. All that combined could explain why sanding between coats is not necessary for Toplac Plus. But these are just my theories based on looking at their chemistries (and I’m not a chemist). For a real answer you’d probably do better asking the Interlux folks.
Doug,
Wet sanding varnish is totally a done thing. Just use fine sandpaper (I like #400) and a light touch and make sure that it’s dry before you apply the next coat. Wet sanding does a really nice job of keeping the dust under control. With Schooner, there’s also a change in feel if you sand through the varnish into the epoxy which is really convenient.
Laszlo