Oil-varnish finish repairability

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Kary Karahadian
Posts: 121
Joined: Sat Jan 07, 2012 6:34 pm
Location: California

Oil-varnish finish repairability

Post by Kary Karahadian »

To what extent are oil-varnish finishes repairable? My finish experience is limited to traditional FP with plain shellac, and RoyalLac. 2 ends of the spectrum. Where does oil-varnish fall on this spectrum?
Alan Carruth
Posts: 1264
Joined: Sun Jan 15, 2012 1:11 pm

Re: Oil-varnish finish repairability

Post by Alan Carruth »

'Varnish' is like 'wood'; it's the name of a class of materials that can differ a great deal from each other. That said, in general, it's harder to do repairs and touch-ups on oil varnish finishes than on things like shellac or regular nitro.

Oil-resin varnishes harden through an oxidation reaction of the oil content with air. The cured varnish is not the same as the stuff in the can, and won't usually dissolve in the same solvents. It's an advantage in durability, but makes it hard to do touch-ups. With a solvent release finish like shellac or nitro the new stuff melts it's way into the old coating, but that doesn't happen with varnishes. Usually, at very least, you need to do a thorough sanding of the surface to get the 'surface energy' up so that the new coats will stick. It's often easier to re-coat the whole surface than it is to do a spot touch-up.

CA will bond well with some oil varnishes, and can be used for spot touch-ups and as a a drop fill during finishing. They often don't polish up quite the same, though, which can produce visible differences in the surface, so I usually only use that sort of thing as a filler in the early coats, with enough build afterward to preclude sanding through (knock wood).
Kary Karahadian
Posts: 121
Joined: Sat Jan 07, 2012 6:34 pm
Location: California

Re: Oil-varnish finish repairability

Post by Kary Karahadian »

Thank you Alan. Is it best sprayed, or does it lend itself to brush application as well?
Alan Carruth
Posts: 1264
Joined: Sun Jan 15, 2012 1:11 pm

Re: Oil-varnish finish repairability

Post by Alan Carruth »

I've always brushed oil-resin varnishes. As with anything, it's a skill that takes a little time to acquire. I don't have a spray rig or a suitable space, so that's out anyway.

Because of the way they cure, recoat times are very important with oil varnishes. Ideally you want to put the next coat on before the prior coat has cured too much, to assure a good bond. However, if you rush it too much the brush will drag and it becomes hard to level the coat out. Waiting too long between coats, or not properly sanding in between, contributes to 'witness lines'; areas of harder and softer varnish when you're doing the final polish that can be visible, and hard to get rid of. In extreme cases of waiting too long, the new coats can simply peel loose like a bad sun burn.

On the plus side, oil-resin varnishes can be extremely durable, chemically stable (unlike, say, nitro), and quite beautiful in the way they bring out the color and 'light' of the wood.
Kary Karahadian
Posts: 121
Joined: Sat Jan 07, 2012 6:34 pm
Location: California

Re: Oil-varnish finish repairability

Post by Kary Karahadian »

Alan, thanks again. I've heard lots of positives regarding oil-varnish finished guitars. Never had one in my lab however.
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