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Bummer. I don't know, but have some ideas...
- The rubber is starting to decompose and somehow stains/reacts with the finish.
- The vacuum is pulling something aromatic out of either the rubber or HHG that stains/reacts with the finish.
- The finish has something in it that is reacting with the otherwise good rubber.

Is the stain only under the rubber area, and not seen in the area in between the rubber and the bridge? If so, sounds like the rubber or finish. If not, something in the HHG?

Some tests to isolate variables...
- Sand off a little finish to see if the stain is just on top, or through.
- In another area of the top (assuming the entire top will be sanded and refinished), do a mock bridge glue up without HHG, with just enough vacuum to keep the rubber in contact. Check it, then repeat in a somewhat different position with full vacuum.
- Replace the rubber or cover with a different rubber film, repeat.

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David Malicky

I have not found any value in vacuum clamping longer than 20 minutes, perhaps it is the extended contact time with the rubber which may be deteriorating as David suggests.
Some rubber does not play well with Nitro for extended contact.

I think that the vacuum is pulling unset resins from the wood towards (or up against) the lacquer.

I've seen the resins pulled out of softwood support blocking while using a vacuum chamber to remove bubbles during investment casting. Betcha' a dollar that's what's happening here.

cheers, Steve

Wouldn't that only affect the area INSIDE the rubber seal rather than just the area in contact with the seal?

I would suspect plasticizers being pulled out of the rubber and into the finish. Replace the rubber with new.

The stain might fade over time but if not, you could hide it by spraying a light toner on the top.

I thought nitro was famous for reacting with rubber, like how it leaves marks where the neck and body rests on the cheapo guitars stands. If you both got your vacuum jigs at the same time, could it be that the rubber is decomposing at the same rate, as has been mentioned, thus causing you both trouble at the same time?

Why not just be safe and kick it old school with a Fox style clamp system when you use nitro?

Or, for that matter, lay down a piece of Saran wrap over the area?

some modern rubber substitutes will interact with nitro . For that reason I am careful on where i use my vacuums. Like the other poster , 15 to 20 minutes is all you need the vacuum helps to get the water out of the glue and cures faster.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Hi Filippo,

How hot and humid is it in your area? Maybe the current heat and humidity might affect the rubber material from your vaccum clamp, or the finish is more prone to react, or both...?

Just trying to find what might be different from the other times you did this without problems.

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

If you are really wanting to know whether it's a reaction between the bag and the nitro, or resins drawn towards the top surface, you could put a small portion of the bag in tight contact with the top, perhaps where the fingerboard or pickguard will cover. After six hours you should know. With no vacuum on, if the bag is at fault, there should still be a hint of discolouration.

cheers

I'm not trying to be facetious here, but you may try this over the whole top. Step by step. Considering it's your sons guitar you still want it to look the best it can.

I personally have never heard of vacuum causing any reaction with lacquer at all and first suspected the rubber itself. Looks like the shading goes all the way to bridge itself.

Huhm! Weird!

Have you contacted McFadden abut this?

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

Can you detect any compaction of the finish? If the finish film has been compacted thinner in that area than the rest of the film it will refract light differently. Could also be that the vacuum has somehow accelerated the aging process of the lacquer in that area. Try some UV light and see if it fades. Or it could be a reaction between the equipment and the finish as others have pointed out.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Has anyone who had this problem found a fix short of refinishing?

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Good judgement comes from experience. Experience comes from poor judgement.

Are you talking about the thread starting in mid-conversation? I don’t see the original post.

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For the times they are a changin'

- Bob Dylan

OP left the forum in a huff and took all his posts with him. This is a zombie thread from 2012...

Ed, You're crackin' me up

All in a day's work!

Oops...did not make the connection. Our monthly diner breakfast is this Saturday ...all of the usual suspects should be there (the boss, Mr. Morelli, Mr. Verhoeven), so I'll have to berate them re: the 'Swiss Cheese Thread' effect. There are times when I think it might be fun to conference in a few of the other personalities involved and just sit back and watch the fireworks.

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For the times they are a changin'

- Bob Dylan

Meh. I find pointless confrontation inhibits data flow.

I find that the effort to hike around to the back side of an issue is usually well worth the difference in the view, but to continue the analogy, designated scenic overlooks have their own charms for those that lack a certain fondness for a sometimes challenging bit of walking.

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For the times they are a changin'

- Bob Dylan

"There are times when I think it might be fun to conference in a few of the other personalities involved and just sit back and watch the fireworks."

Fireworks are nice as long as you are not in the fire fight. It never bothered me to read discussions where people expressed strong opinions, but when they devolved into name calling instead of agreeing to disagree, I could understand why the moderator would step in.

Someone once asked me if I could take a joke. My response was "yeah - as long as the joke is on someone else!"
Where is Zombie boy when you need him - oh! my bad.

At least for this past weekend, Mr. Zombie Luthier was setting his Brazilian/adi 000-42 and D-42 12 fretter's necks, and shooting the sealer and flash coats on the second of his 355 repros to pass through this shop.

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For the times they are a changin'

- Bob Dylan

Sanded and buffed, stain is gone. LMI is aware of this issue and are experimenting with different materials.

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Good judgement comes from experience. Experience comes from poor judgement.

Had a nice discussion on vacuum clamping at the monthly breakfast Saturday re: zombie threads and vacuum clamping. In addition to the usual disparity between clamping pressure using vacuum and other methods, it appears as though length of time under vacuum plays into the staining issue...the longer under clamp, the deeper the penetration of the stain. The use of tissue or other permeable barrier between the bag and the guitar was also discussed as a precautionary measure.

To riff on the zombie theme of the thread, I'll offer the following manifestation of the web's fascination with uninformed speculation and unfounded assertions:

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For the times they are a changin'

- Bob Dylan