Repairing a separated back

[Arlen]

I've mentioned more than once that my '21 A has a shrunken back that has separated from the side - probably about a 6" long open joint. I bought the mandolin having seen pictures and was aware of the separation. I got a really good deal and feel good about the purchase, and I can't afford better for now. I've thought quite a lot about how I'll go about repairing this when I do. It'll be a while still before I get to it, but I thought I'd run my thoughts by you and ask some of the questions that I have. I'll include a couple pics that will help explain what I'm facing (not as clear as I wish they were). I want to build one day, so this is a good exercise for me.

First, I figure I'll plan to re-refinish the back and sides as a part of this. They have been refinished already so I won't be hurting the value any more than has already done, and it will help me hide the repair.

I plan buy a seam separation knife from StewMac and use that in combination with heat to take the back off, clean things up including removing the existing finish, and reattach the back w/ hide glue. I originally thought that once the back is on, centered as best as I can, I would route the edges for a thick wood binding to take care of the shrunken issue. However I'm thinking now it would probably be easier to add the wood 'binding' and enlarge the back while it's off, trimming it to size once attached. I don't see justification to making a new back, as I only need to add approximately 1/16" worst case around the perimeter (it looks like a binding ledge in places, but I'm pretty certain it's not).

So, my questions:

- I don't know what kind of finish was used when the back and sides were redone. How can I determine that, and what's the best route for removal? What is the best way to protect the face during the removal?

- I am guessing the back was reattached at some point, and didn't shrink in place as the worst locations are securely attached, but I may be wrong. If it's original, I'm sure it was attached with hide glue, and heat w/ the knife will allow me to remove it w/o problem. If another glue was used is the heat / knife combination my best bet still?

- There appears to be overspray on the front binding, and perhaps the top was oversprayed as well. I don't think I want to change the top, so will mask it off for the refinish. Can I safely scrape the original bindings clean after refinish, or should they be protected w/ something (I think I read somewhere that old bindings deteriorate if exposed).

-What kind of tonal / volume change can I expect this repair to bring? I assume there will be a change as the back can't be doing it's job fully right now. Regardless of whether a change will occur, I need to fix it as I'm concerned one day I'll catch the open seam on something and break the back.

I've read quite a lot on building, have spent a fair amount of time on Frank Ford's site, and have a reasonable amount of woodworking experience, so I think I'm on the right track, but would appreciate any input on my approach to this specific repair, and help with my questions.

Thanks

http://www.breezechasers.com/images/0-misc/mando-back.jpg

[Arlen]

Follow up question:

What type of finish should I use for my re-refinish. I'm tempted to use the water-based lacquer so don't have toxic / explosion concerns working in my basement. Is this a quality finish that's easy for a rookie to use?

[sunburst]

I plan buy a seam separation knife from StewMac and use that in combination with heat to take the back off,


Fine, but I prefer a good quality putty knife from a
hardware store. The handle feels better, and the blade
is stiffer, yet thin.


clean things up including removing the existing finish, and reattach the back w/ hide glue. I originally thought that once the back is on, centered as best as I can, I would route the edges for a thick wood binding to take care of the shrunken issue. However I'm thinking now it would probably be easier to add the wood 'binding' and enlarge the back while it's off, trimming it to size once attached. I don't see justification to making a new back, as I only need to add approximately 1/16" worst case around the perimeter (it looks like a binding ledge in places, but I'm pretty certain it's not).

If you make a jig that works like the one Frank Ford
has in frets.com to push the rim into shape, and then
center the back, I don't think you'll need "binding".
I've glued those backs back on when they were pretty
shrunken, but the "gap", equalized all the way around,
wasn't bad. I'd check the fit first before cutting away
any back material.

So, my questions:

- I don't know what kind of finish was used when the back and sides were redone. How can I determine that, and what's the best route for removal? What is the best way to protect the face during the removal?

Check it with acetone or lacquer thinner. That will
tell you if lacquer. Physical removal is the best way
to remove the finish without any danger to the top.
(scraping and sanding)

- I am guessing the back was reattached at some point, and didn't shrink in place as the worst locations are securely attached, but I may be wrong. If it's original, I'm sure it was attached with hide glue, and heat w/ the knife will allow me to remove it w/o problem. If another glue was used is the heat / knife combination my best bet still?

Heat might help, and it might not, but the thin knife
is the right procedure. If the knife isn't
popping the glue line, I usually sacrifice the linings
inside the instrument by directing my putty knife into
them slightly rather than do damage to the plate or
sides. I can replace the linings if I have to.

- There appears to be overspray on the front binding, and perhaps the top was oversprayed as well. I don't think I want to change the top, so will mask it off for the refinish. Can I safely scrape the original bindings clean after refinish, or should they be protected w/ something (I think I read somewhere that old bindings deteriorate if exposed).

Probably should leave the bindings alone, unless they
are just too unsightly. Or, if you clean them, refinish
them with shellac. If you use orange shellac, they'll
be about the right color. Chemical stripping and
refinishing with lacquer is what seems to ruin old
bindings.

-What kind of tonal / volume change can I expect this repair to bring? I assume there will be a change as the back can't be doing it's job fully right now. Regardless of whether a change will occur, I need to fix it as I'm concerned one day I'll catch the open seam on something and break the back.

It'll sound better. That's the only prediction I'll
make.
The original finish would have been spirit varnish. Shellac, basically, maybe with other stuff in it. I would refinish with that. It's non toxic, but the alcohol solvent is a slight fire hazard. Not like lacquer thinner, though. I would be comfortable with shellac or spirit varnish in the basement, and it would be closer to the original.

[Arlen]

Thanks John for the detailed answers, you've been a huge help.

I assume by "make a jig" you're referring to Frank's use of alum. angle for demountable bench clamps? That's my plan, but it was good to review that article, it's been a long while since I read it. The reason I think it's a shrunken back vs. just distended sides or poorly reglued back is the difference in size at the little neck heel portion of the back. I'm glad to hear I probably won't need to add to the back, but think I'll still probably have to add some material at the neck heel to clean that up at least.

I guess I'm comfortable with avoiding chemicals and simply scraping and sanding, especially if that will help protect both top and bindings. Sounds like a lot of elbow grease is in my future, but I don't want to damage the parts I'm trying to leave original.

Sacrificing the linings if a joint won't let go is something I hadn't considered, thanks for that.

I've not used shellac, but want to learn to work with it, so here's my chance. The only article I found on Frank Ford's site that uses shellac is his French Polishing article. (He's got a big site, I may have missed it). Do I use the same technique to apply the shellac he's used for the FP, but use another polishing technique? Or is FP the way to go?. How is color added to a shellac finish?

The bindings aren't unsightly so to speak, but are quite yellowed. There is a 4" portion that I originally thought may have been sanded away during the 1st refinish, but have come to realize has darkened so much that it blends with the 'pumpkin' top. I should leave it alone, but probably wont. I'll likely scrape, then protect it as you've suggested.

Quote: "It'll sound better. That's the only prediction I'll make."
LOL, got it. Worth doing to preserve the instrument, and better is better.

Thanks again John, great info