Detecting Scratches

[John Maddison]

Hi All

Open to suggestions for tried & true techniques to detect miniscule scratches, particlarly in tops, prior to finishing.

Have tried a wash with shellite (naptha) or methylated spirits after final sanding to 320 grit, even a pre-finish sealer coat of shellac, but to this day am frustrated with still finding minute scratches, which are an absolute pain to remove, after having applied the first coat of finish (usually hard wax oil).

Thanks

[kiwigeo]

Shellite generally works for me on most of the spruce tops I work with. What's the finest grade of sandpaper you work your tops with?

I find a jewellers 10x lupe is handy for picking up for close up examination of a top prior to finishing. I also use it to check out my french polished finishes for lint and crazing.

Cheers Martin

[John Maddison]

HI Martin
I generally only go to 320 grit on all surfaces, sometimes to 400. Probably my biggest fault is impatience - having spent so long building the instrument I tend to rush the sanding phase and thinking I've achieved a surface ready for the finishing. Thanks also for the tip about the magnifier. Cheers!

[Allen]

The trick to getting all the scratches out is a thorough sand of each and every grit up to P320 or P400 at the absolute most. After that you will compromise adhesion of all finishes. Do not jump more than 100 points between grits. In the neighbourhood of 50 to 60 points is best if you can. Each of those grits refines what was left previously. Your first grits level out irregularities, and the rest are just used to take out those scratches. It goes quite quickly if you work your way through. However if you jump from say P150 to P320 you will be there all week and still likely to miss some.

And remember, finishing the instrument is at least 1/3 of the work. If you want it really flat and shiny and are not all that experienced then plan on it being as much as 2/3 of the work. You can't rush it. Make it a time to meditate. I can sand all day long and not realise that 8 hours has gone by. It's best to make it a time to enjoy by putting on a favourite album, having a glass of nice wine etc, rather than something you dread.

You can give a wipe with a damp (not wet) cloth to raise the grain. Dry with a blow drier if you like to speed things up, and then proceed with the next grit.

Enjoy John. I know you like a nice glass of red, and good music.

[John Maddison]

I know you like a nice glass of red, and good music.

You know me too well, Allen ... thanks for a concise reply!

[auscab]

With furniture when it's got to be spot on for a french polish job I turn off the lights and get a torch ,rest it on say the table top so the beam shines accross and casts shadows ,it shows up all marks,dips tiny scratches.
cheers Rob

[Clancy]

I hate sanding, painting and plumbing.
Luthiery requires perfection in two of these 'horrors'.
If someone ever asks for an 'S-bend' on their guitar I'm going to quit making these things

[kiwigeo]

With furniture when it's got to be spot on for a french polish job I turn off the lights and get a torch ,rest it on say the table top so the beam shines accross and casts shadows ,it shows up all marks,dips tiny scratches.
cheers Rob

All fine until your neighbours call the police to report burglars prowling around your workshop with a torch!

[auscab]

That would actually be a good thing, lol ,they might have caught the buggers that knocked a 1.5 x 1.5 meter hole in the wall and filled a truck with my Old Man's furniture, pity they wern't on the ball that night,that was about 19 years back.

They were on the ball another time, we had left for the Easter break and a polisher had left a bunch of rags soaked in linseed oil and turps out the back on a table ,We left Thursday ,the rags caught fire Sunday afternoon, the neighbours were having a beer on their balcony, and rang the fire brigade, plus jumped the fence and put it out .if had not been noticed it would have taken the work shop at least.

[tim mullin]

Open to suggestions for tried & true techniques to detect miniscule scratches

I don't think there's any replacement for examination under bright sunlight. I have tons of light in my shop (ageing eyes), but I still find that bright sun makes all surface features 10x easier to detect. So, meditate and sip your wine during your systematic surface prep, but don't declare it perfect until you've had a good look in the (bright) light of day.

[WaddyT]

FWIW, finishing Guru, Joe White says always use incandescent light as it enhances scratches. Fluorescent light hides scratches. He didn't mention sunlight!

[liam_fnq]

As a general rule if you can't see it under sun light you won't find it under artificial light. Even the MCG is not as bright at night than it is during the day. I haven't tried the torch trick.
Incandesent lights give a sharper light with stronger shadows. Florescent and LEDs give a softer light with softer shadows. Incandescents are much better work lights I reckon. Bit hot though on a 40 degree day in cairns.

[Kamusur]

Good tips Allen ty

Steve

[John Maddison]

All great advice, y'all ...

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