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roby
 Post subject: Nitro over french polish
PostPosted: Fri Jul 24, 2009 4:40 am 
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First name: robin
Last Name: courtenay
City: andover
State: hants
Country: uk
Focus: Build
Status: Amateur
Hello all i finished my first build with french polish about three weeks ago with good results and glued the neck on, everything looks good but am dissapointed in how easily the finish scratches,i knew it was a delicate finish but expected it to be harder,i haven't glued the bridge on yet and wondering if it is possible to apply a nitro finish on top of french polish (Behlens aerosol lacquer) advice would be appreciated thanks all Rob.
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Kim
PostPosted: Fri Jul 24, 2009 5:23 am 
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Location: Changes when ever I move..Australia
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First of all Rob, FP takes it's time to polymerize. In fact it takes quite a bit longer to harden up than nitro and that can take about 4 weeks before it's hard enough to buff out. So really, before you pass judgement on the durability of your finish I think you should give it time.

Having said that (and I am definitely no pro in this area so take it as you will) shellac is often used as a sealer or binder under nitro. My 'guess' is that with a clean up using naptha to remove any oils followed by a fine light sand, you should be good to go. But you would do well to wait for confirmation of this from someone more experienced with these finishes.

Cheers

Kim
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roby
PostPosted: Fri Jul 24, 2009 6:18 am 
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First name: robin
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Thanks for the reply Kim maybe i should give it a couple more weeks and see what it's like then.Thanks Rob
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Alain Desforges
PostPosted: Fri Jul 24, 2009 9:09 am 
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Many people do seal wood under nitro with shellac, but the shellac has to be 100% dewaxed. I don't know what kind of shellac flakes you used, but I'd make sure there is no wax in there first. If so, sand back to wood. Shellac comes off very easily, actually. I know the prospect of having to sand a whole guitar back to wood is not very appealing, but if you apply nitro of wax, you'll have to anyways and then it'll be twice as hard. If you do sand back, consider spraying your shellac (if you're set-up). That's what I'm doing on this latest guitar I'm finishing and it's quick and painless.

As for cleaning up prior to applying nitro, I'd use Methyl Hydrate (Denatured Alcohol) as this is what is used to clean up oil while French Polishing.

Kim is right, it might take a while for your shellac to harden to its full potential. But even then, it will never rival the modern finishes for protection.

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Alain Desforges
PostPosted: Fri Jul 24, 2009 9:16 am 
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Alain Desforges wrote:
Many people do seal wood under nitro with shellac, but the shellac has to be 100% dewaxed. I don't know what kind of shellac flakes you used, but I'd make sure there is no wax in there first. If so, sand back to wood. Shellac comes off very easily, actually. I know the prospect of having to sand a whole guitar back to wood is not very appealing, but if you apply nitro over wax, you'll have to anyways and then it'll be twice as hard. If you do sand back, consider spraying your shellac (if you're set-up). That's what I'm doing on this latest guitar I'm finishing and it's quick and painless.

As for cleaning up prior to applying nitro, I'd use Methyl Hydrate (Denatured Alcohol) as this is what is used to clean up oil while French Polishing.

Kim is right, it might take a while for your shellac to harden to its full potential. But even then, it will never rival the modern finishes for protection.

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Alain Desforges
PostPosted: Fri Jul 24, 2009 9:17 am 
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Sorry about that. I guess I was 'quoting' and not editing...

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