Mask the bridge glueing area

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Dave M
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Mask the bridge glueing area

Post by Dave M » Wed Jan 24, 2018 12:08 pm

I am finish sanding a build - slightly finicky but satisfying because it means you are nearly there. And I keep seeing pictures on this forum and others of the bridge glueing area masked off.

Now I know that G and G don't do this but I also know that the process of removing the finish is fraught with danger. You can easily scrape beyond the outline, you can easily dig into the soundboard. (And you are made aware of how damned thick your supposedly light coating on the top is!)

I have thought of masking a little less than the bridge footprint so that the majority of the glueing area is free of finish then cutting to the outline which might make taking the finish up to the line a bit easier.

So I wonder what people's thoughts are on this...?
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Re: Mask the bridge glueing area

Post by johnparchem » Wed Jan 24, 2018 12:30 pm

I have been go back and forth between no masking and masking most of the bridge area and clearing to a scribed cut. Of those two options I am leaning toward mask most and cut to the line. My experience: If you mask inside 2-3 mm and after finishing you scribe the bridge outline with a sharp scalpel, It is easy to clear the 2-3 mm with a chisel. When working from the inside the chisel catches on the finish amd chips it right to the scribed line. At one time I thought the center is safe to clear so why bother masking anything. Scribe as described above with a fresh scalpel blade. Clear the inside and then chisel or scrape to the line. I did this a few times and found that without the sharp edge of the masked off finish it is possible the chisel could ride up over the finish skate past my scribed line maring the finish.

Having said all of that on my next instrument I am going to apply masking tape to the top, position the bridge, and then carefully scribe the masking tape with a scalpel. Then I will lift off the waste tape and leave a complete mask. I will decide what to do after I finish and remove the mask and see what I have.

simso
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Re: Mask the bridge glueing area

Post by simso » Wed Jan 24, 2018 12:32 pm

Taylor do that, Basically, they micro route, I scrape around the bridges perimeter to allow a small step to exist that will accomodate for the paint going fractionally under the bridge. 2mm is more than sufficient.

Then youy just mask that area of the top when painting

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demonx
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Re: Mask the bridge glueing area

Post by demonx » Wed Jan 24, 2018 1:15 pm

What Steve said, however to take it a step further, rather than just mask, I have stickers I make up with two holes which help me with alignment.

I position the bridge pre paint which has the two holes in it and then I drill the holes into the top ready for post paint mounting.

After paint I razor blade the sticker silhouette which is 2mm smaller than the bridge silhouette, peel it off, then the holes allow me to mount the bridge post paint without needing to align everything up again.
IMG_6037.JPG

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Re: Mask the bridge glueing area

Post by simso » Wed Jan 24, 2018 1:31 pm

Noiiiice
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demonx
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Re: Mask the bridge glueing area

Post by demonx » Wed Jan 24, 2018 1:38 pm

simso wrote:
Wed Jan 24, 2018 1:31 pm
Noiiiice

Thanks, I felt like a genius when I thought that up!

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Mark McLean
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Re: Mask the bridge glueing area

Post by Mark McLean » Wed Jan 24, 2018 10:11 pm

Yep - I think you can consider yourself pretty clever for that suggestion. I am going to give that a go.
Thanks for sharing - and good question Dave.

Dave M
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Re: Mask the bridge glueing area

Post by Dave M » Thu Jan 25, 2018 7:02 am

Thanks everyone, there are some great ideas there.

I realise it is a question without a straightforward answer, but I am trying to tease out the pros and cons. As a really quite poor sprayer (of nitro) I have to do a lot of sanding and I can see that having that obstruction in the middle of the soundboard is going to be annoying. Up until now I haven't masked so a clean sanding sweep is good. I just don't have a feel for how annoying/interfering a piece of tape might be.
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Allen
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Re: Mask the bridge glueing area

Post by Allen » Thu Jan 25, 2018 7:31 am

When you mask off it's a pro and con. The con is that the finish will create a ridge next to the maksing material. If your finish is thicker than ideal this ridge will need to be dealt with in one way or another so the bridge will sit well.

You need to scrape a small amount away to the outside of the bridge foot print, or ease the the outside edge of the bridge. I've seen builders do both. There was even a tutorial on the OLF about routing a wisker off the underside outside edge of a bridge to deal with this.
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demonx
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Re: Mask the bridge glueing area

Post by demonx » Thu Jan 25, 2018 8:35 am

Dave M wrote:
Thu Jan 25, 2018 7:02 am
As a really quite poor sprayer (of nitro) I have to do a lot of sanding and I can see that having that obstruction in the middle of the soundboard is going to be annoying.
Sanding around anything is annoying and with guitars there's plenty to sand around, however maybe some painting tips may be in order so there is less sanding for you to worry about.

First off, sanding can't really be avoided, as there will be plenty of sealer coats until everything is flat. Unfortunately there's no straight answer as to how many, it is however many it takes, but once it's flat then things get easier. The trick to having a flat surface (other than sanding) is getting the paint to flow onto the surface and flatten itself out, self levelling I guess.

Tips I've found over the years are, and these will depend on paint product however generally speaking I do the same be it enamel, acrylic or 2k.

Use a thicker tip than you'd use for colour. For example I use a 1.4 on base coats and a 1.8 on clear coats, it helps if flow better into the surface. Good flow means less sanding and better finish.

It takes a lot of practice, but you need to get enough on at once so that it's thick, but not quite enough to run.

Spray the very first coat way too fast, so it's virtually just a mist onto the surface, this is called a tack coat, give it a minute and then spray your real first coat, this will help prevent runs.

If you get a run you're either spraying too slow or spraying too soon between flash times. Ignore flash times on data sheets, they are written assuming you're spraying in a pro heated booth. With flash times I extend each time as I go, for example: tack coat, 30-60 seconds later first coat, flash for 5min then second coat, flash for ten minutes before third coat, if you do a forth it'll be after 20min and so forth, as the thicker it gets, the more time it needs.

Spray wet to wet. Make sure to spray in an order so that you are always spraying next to your previous wet coat, so that the paint can flow into the last pass you sprayed.

I move quickly with the gun and typically use a 30-50% cross over, this helps the paint flow.

Where there are lumps and bumps (like a taped up bridge) , ignore they are there and spray as if it's a flat surface. This will get a better flow.

Hold the gun close enough (~15cm +-) that the paint is flowing on wet and not getting a mist, as once the paint leaves the gun it starts to cure and a common problem is people spray too far away and the paint is really falling into the surface and not spraying onto the surface.

Air pressure gauge at the gun, not at the compressor. Too many people regulate the pressure at their compressor, by the time the air gets to the gun it drops several psi and you get the same effect as holding too far away where the paint isn't blasting onto the surface, it's falling on.

Anyway, hopefully if you get all this into your spraying habit, you won't have so much sanding to do!

Dave M
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Re: Mask the bridge glueing area

Post by Dave M » Fri Jan 26, 2018 8:37 am

Allen yes thanks I am leaning towards masking just inside the bridge outline then scribing round it and removing the last mm or so as per John Parchem's suggestion.

Allan thanks for taking the trouble to share your experience. Those are things I am trying to implement - particularly the speed and overlapping. I shall be spraying in the British winter so flashing off is not going to be happening too quickly. I have to take the sprayed units back into the warm and dry to stop problems with moisture. It seems to work OK.

I finished the sanding today so if the weather allows I'll get the first coats on tomorrow - always a nice feeling!
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Re: Mask the bridge glueing area

Post by blackalex1952 » Fri Jan 26, 2018 11:26 pm

I'm not the greatest sprayer around, I have a cheap gravity feed gun and a really old gun which I restored. I have had some success with nitro, warming it in the pot by placing the pot (on the gun) in a container of hot water. You could give that a try in cooler weather, and heating the guitar with radiant heat lamps. Too risky with nitro vapour in the air to use a low heat gun or hairdryer as the nitro is extremely inflammable. I use spray booth filter fabric in a box I made with a plastic booth and a non arcing suction fan behind the filter. I use an insect mesh screen and spray outside. The heat lamps keep the air and the instrument warm. But before I made my budget booth I would warm the guitar directly before the first coat and hot water treat the nitro. After that more gentle warming so as not to damage the nitro while it flashes off. Initially to take the chill off the instrument then to maintain the warmth. Or as you do, back inside the workshop, but be careful of the off gassing. Nitro shouldn't be sprayed below 20 degrees Celcius. Cheers! Ross
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