French Polishing Curing problems

[bearskindrum]

Hi most learned French polishing Gurus.... I have issue with my French polishing. Having filled a collapsed section of my rosette many weeks ago using a "jelly drop " technique as suggested by Brian Howard in another topic, I have a section around my rosette and upper bout where the polish was probably put on too thick (in my haste to progress the build...I know, I know... not supposed rush) and now is taking a long time to harden( last coats put on 2 weeks ago) as I found out when working on the fingerboard tonight when I managed to put some compression marks in the finish even underneath a high grade cardboard caul which I had made to protect the top of the guitar,which I thought was hard...OK so I need to get the top hard( well DUH!)... so should I

1)wait and see how the top settles over the next few days weeks and assess/repair the damage then?

2) Sand back/level very lightly to get rid of the compression marks and by exposing the finish a bit more, this should let the finish underneath harden a bit more before re-polishing and spiriting off again?
I still need to attach the bridge and I guess that I could do the prep work for this this (as the bridge area/ lower bout area is hard) while the rest of the top hardens around the upper bout? exposing this bridge area would also allow me to see how thick the finish is in case it is too thick and the top finish needs to be thinned all over? I would do this before finally attaching the bridge

Guitar building room is at about 50% humidity but gets hot during the day while at work 30 degrees Celsius( middle of our Summer). Shellac that's been used is fresh from LMI and dissolved in alcohol as recommended.

As Always you advise would be most gratefully received

Regards Mark

[B. Howard]

Tape and cardboard to protect finished surfaces is a good idea but far from bulletproof. It is possible to damage a properly cured finish through them quite easily with a slip of a file or too much pressure of any type.

You mention knowing the coating is too thick from being built up too fast, this means that in addition to too much material being applied at one time that too little pressure was also used. FP is the art of applying minimal amounts of material under pressure to form a finish. Without the required pressure it is the same as brushing it on. Shellac also doesn't like to be thick no matter how it's applied, the thicker the coating the softer it will be. This really has little to do with drying and more to do with the physical properties of the resin.


If it is truly thick in this area I would advise sanding back a bit, possibly even enough to remove the damage if that is possible and then polishing out with a bit of compound like Novus or Finessit.

As always it ids tough to make calls on what is going on without actually seeing the problems.

[bearskindrum]

Hi Brian, many thanks for your advise...what sequence grade of sandpaper grits would you recommend to use to cut back the defects with before using the Finesse it to polish up the polish finish with? would you also use a sequence of polishing compounds as well i.e. a courser cutting compound to be followed by less aggressive cutting compunds?

By the way, for what its worth, I think that you handcrafted guitars and website look awesome

Kind regards

Mark

[B. Howard]

For sanding out marks and such like this I use wet/dry paper. You may use naptha as lubricant but on FP I find myself using a drop of olive oil as it seems to gum up a bit less with shellac on the paper. It does make it harder to see what you are doing though so naptha may be better for you at this time. I will usually start out at 600 grit and progress 800, 1000, 1200, 1500 then use my polish and a felt block to finish it up. If using Novus, #2 is the only one you should need, haven't used the 3M in awhile so I am not sure on that one.

[bearskindrum]

Thanks Brian I will try this and let you know how I get on

Regards Mark

[bearskindrum]

Hi All, tried what Brian suggested and it seemed to work really well went trough the grits from 400-2000 and polished up with the Novus No2 Scratch remover and it worked really well...Thanks so much Brian...really great adivse and very pleased with the result

Regards Mark