Crack repairs and glue joint lines manifesting through lacquer finish

[Rory]

Hi folks,
Here's a question for you-I am using McFaddens lacquer. I'm using the pore filler they supply, the sealer and either their gloss or matte lacquer. Problem I am having is the lines of crack repairs and glued joints showing up through the dried & cured lacquer. It does not seem to matter how much wood I put behind the glue joint to cleat it, the thin line of the join is always manifesting through the lacquer finish, looking like a long thin straight crack.
So, what do you figure? Is it that i am just not putting on enough lacquer? I am putting on what I figure is quite a lot of coats-varies depending on the wood, but plenty to flat sand and buff without burning through and no pores of the wood are showing through. Built instruments showing this are kept in fairly even humidity and all wood is well acclimatised to my shop. Its is happening on all sorts of wood varieties, not just one type or batch. If the only solution is to lay on more lacquer, then I may just live with it, but I wondered if there is a trick to avoiding these lines showing through? I should mention, at first they do not show through, but within a couple of months they start to show up.

Rory

[Frank Ford]

I suppose this won't be of much help, but I always advise customers that such cracks are likely to be visible in reflected light as the lacquer cracks over them. Sometimes, it doesn't. . .

[Greg Mirken]

And the longer you can wait before buffing, the less the defect will telegraph through the lacquer. If possible, 3 or 4 weeks of curing should help.

[Ry Charters]

Lacquer will sink into "non wood" areas differently than others. Water based glue will shrink over time (especially if the joint isn't tight). If its appropriate, I use CA instead of wood glue to try and avoid this (on some cracks), but there's still a noticeable line.
I know some builders will let their guitars sit around for up to a month before they put finish on them to make sure the glue has had enough time to dry up, and the wood has settled around the joints. I think a week would be plenty of time to wait for this.
Another culprit could be getting the lacquer too hot when buffing. If it "melts" from the heat, it will sink into the instrument again to an extent. I buff out most of my crack repairs by hand to reduce heat.
It never stays completely flat forever, but these little things can help.

[Russell Vance]

Two things I know does this: nitro put on thick with a low thinners ratio: In my early days myself and my peers were learning the ins and outs of nitro and were laying it up too thick and with low thinners ratios and getting all sorts of crazing/checking and cracks as the weather and humidity cycled. A return to factory specs (50/50 ratio) and many thin coats has solved most of out problems (including a reduction in orange peeling and prolonged curing cycles/lacquer pilling).

Secondly, we clamp everything at industrial levels IAW with the glue spec (Titebond) which appears to be twice as much as the general users do (for fear of the old carpenters chesnut - a staved joint) and those who glue up with a couple of rubber bands etc. This works - check out veneer gluelines (non existant lacquer lines on veneer joints) to see what the correct joint preparation and glue pressure can do.
Rusty.

[Rory]

Thanks so much guys for all the feedback. I have been waiting 11/2-2 weeks until flat sand & buff so I'll try longer & see how that goes. Ive only ever thinned my lacquer with some retarder if the humidity is high. It seemed plenty thin out of the can. I am using titebond glue. Rusty, wow- 50/50 mix lacquer to thinner-I had no idea- I'll do it if I have to, but I cant imagine how many coats you need then? I can see that it would make the lacquer very thin though & that might help it to creep into all the crevices better? I've never been able to find 'factory specs' for Mcfaddens lacquer application- anyone know where I'd find that info? The Mcfaddens site only appears to be about selling product- i dont see any support, a forum or any useful application information on there.
Also, if I may hijack my own thread- i am considering a change to water based lacquer- can anyone chime on on how much this problem comes up with the water based KTM? Any worse or better than nitro?
Thanks again!, Rory

[Ry Charters]

KTM....I know guys who use it and get good results. I sprayed acoustics and electrics with it before I had a proper booth, and I'm never going back, even though they came out alright. The finish was prone to clotting in the gun. Even being thorough with my cleaning and straining. I also had problems with buffing in that I had to be much more careful with heat. Its fussy.
The initial wash coat that should be thin enough (3 parts thinner to 1 part lacquer) to soak into the pores and gaps well. Thinning just makes the flow better as it hits the instrument. If you build up too much (to many thick coats) it can crack on you. I imagine you're waiting an hour or so between coats to allow it to dry properly right?
Finish applied in an environment that is an incorrect humidity or temp can definatly crack and shrink as those factors change. You should maintain around 50-60% and keep the temp on the warm side while its drying. Don't want to swell the wood and then have it shrink on you a week after buffing......
Buy the StuMac finishing book if you don't have it already. Its worth it for the recipes alone.... But its full of info and troubleshooting. And its got all the thinning ratios, proceedures, specs, ect that you're looking for. It really helped me a lot.