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Lacquer cracking
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10123&t=46363		 Page 1 of 1
Author:  Freeman [ Mon Sep 14, 2015 9:20 pm ]
Post subject:  Lacquer cracking
I'm in the process of finishing a maple guitar and have been cleaning up some previously drilled holes. Unfortunately the lacquer is cracking and chipping away at some locations. Here is where the end strap button will be located - I put a screw eye in here to hang the guitar, when I removed it the finish cracked as you can see

Image

I also predrilled the pickup mounting holes and several others, a couple of them have cracked in similar (but smaller) fashion.

Briefly the guitar is flamed maple lamination (like a Gibson 335). I sanded to 220, then applied a stain (StewMac) dissolved in alcohol to pop the grain. Sanded back with 220, then a wash coat of nitro diluted 1:1 to seal the stain, followed by a coat of amber (StewMac rattle can). Then 4 coats of nitro (Belden) with the red color, all diluted approximately 3:1, then six coats of clear. The finish has set for three weeks, I'm just starting to do the final color sanding and buffing (which is why its dull). The finish has come off in chunks about 1/4 inch square and seems to be at the first layer. Coats were applied two or three per day, sanded to 320 each day, more or less over a continuous period of time. Temperatures were in the 80's, when I pulled the tape on the binding between the red and clear I got a tiny bit of chipping but also scored around the tape first with an Xacto knife.

I've been able to drop fill the chips so far and hardware will cover them but I'm worried about the long term stability of this finish. The guitar will leave my care (and actually, this country) so I don't want the owner to have any issues. If its going to be a problem I'll strip it and start over - obvious I don't want to do that.

Comments would be appreciated
Author:  Jim Watts [ Mon Sep 14, 2015 11:41 pm ]
Post subject:  Re: Lacquer cracking
I'm thinking this is an adhesion issue, I didn't see in you finish schedule a first layer of vinyl sealer or shellac.
These first layers of vinyl sealer or shellac act as a tie coat improving the bonding characteristic of the finish.
It'd be interesting to hear the opinions of others.
Author:  B. Howard [ Tue Sep 15, 2015 7:07 am ]
Post subject:  Re: Lacquer cracking
As Jim stated, sealer is an important component of a finish system and serves to bond the film to the substrate.

Another potential problem is the layer of rattle can lacquer at the bottom of the film stack. Mixing different brands of finish can often cause these type of problems.

Another leading cause of delamination is dust or contamination between coats. How did you clean after sanding between coats?

Could also be your red shading coats. Too much color and not enough binder can make that layer of the finish brittle and chippy.

There are other possible causes too. But to get to the real question of stability down the road.....that is a tough call to make from here based solely upon your description and a single photograph. This is a judgement call you will have to make. There are no non destructive ways to test for film strength and bond.
Author:  Freeman [ Wed Sep 16, 2015 8:54 pm ]
Post subject:  Re: Lacquer cracking
Thanks Jim and Brian/ No sealer - I haven't used one in the past and with the stains I thought it might muddy things. I've never quite understood shellac or vinyl sealers - sometimes I'll put a coat of shellac (brushed out of a can) on spruce before I route binding channels but it seems to color the wood and I usually try to sand as much as possible before shooting the lacquer. Anyway, I guess I need to explore that more.

Some of Brian's other questions - yes I worried about the rattle can plus Belden layers but figured that nitro was pretty much nitro. That might be wrong.

Sanding each day to level any orange peel or dust, then either vacuum or blew it off with the compressor, then wiped with a rag before the next coats.

The red color coats were mixed a few drops of StewMac Colortone dye in four to eight ounces of lacquer, then diluted with thinner. I don't think the color is too heavy.

I've done a few of these before without any problems - this one just kind of jumped up and bit me. I've been buffing it out - seems to be fine. Will carefully drill the rest of the holes and mount the hardware - if I have problems I may tell the person that I'm building for that I need to strip and refinish - I don't want him to have any problems. We'll see, and I'll let you know.
Author:  philosofriend [ Wed Sep 23, 2015 8:31 am ]
Post subject:  Re: Lacquer cracking
If you spray shellac and make sure it is not the "orange shellac" but the "white" it will not color the wood much. It is worthwhile to make sure it is really fresh so that it will dry fast and hard and be easy to sand.
Look around your entire work area for any way that oil, wax or silcone could get on the guitar. Air compressors and air tools can send invisible oil fog around. Armor all can spread its evil by touching something that touched something else that touched something... well you get the idea.
Blowing the dust off with highest pressure air between coats not only helps adhesion but it helps the grain of the wood look less muddy. But if your compressor is leaking oil....
The ancient cure for nitro that is too brittle is to add a drop or two of castor oil to a pint of spray. I've done this on model airplanes but I suppose it would work on a guitar too. Not all nitro is equal, and manufacturers brag about the success of their own plasticizers that they add. It is possible for a bad batch to leave the paint factory.
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