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I am getting ready to put my sanding discs on my MDF radius dishes. Do most seal them with something or just use them raw?

Thanks.

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Brad
Avon, OH

Hi Brad,

I'm getting ready to do the same thing. I put a seal coat of shellac over mine although it's probably not required.

My question is this: If I decide to go with a vacuum clamping system in the future, I see a bunch of guys that cut channels into thier dish. Is this required, and if so, do you use a seperate dish to sand, or cut channels through the sanding disks, or other?

Cheers!

John

John...my guess is that you would need 1 dish to sand with and another dish that is modified for vacuum clamping. However, lets see what the experts say. I am curious myself because I have vacuum that I would like to use in the future.

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Brad
Avon, OH

i use a separate radius dish cut to an approximate guitar shape for clamping as it allows the frame to be smaller. no grooves.

i also sealed with zinnsers seal coat shellac.crazymanmichael39110.9787615741

Wipe on gel of sealing urethane is what i used on mine. It worked very well and the radius not moved on me at all.
But I also took about a 16th of an inch off of the flat side as well.
Hopefully Tracy chimes in and lets me know if this is correct.
Even though MDF is not supposed to move, it will over time if you cut into it like we do building our own dishes. As I understand it, MDF is created using TONS of pressure (and whole lot of nasty chemicals, but thats not vital to this story). When we remove the radius portion of the dish, some of this pressure is relieved and it can warp. A solution to this is to relieve a bit of pressure from the opposite side of the dish as well to balance it out.
Sorry if this is straying a bit, but if the goal is to prevent warping, I think this is a good idea as well.
-j.

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“If God dwells inside us like some people say, I sure hope He likes enchiladas, because that's what He's getting”
-jack handy

I used two 3/4 MDF dishes glued back to back. One side for sanding, the other for clamping. One double sided dish for the top, another for the back. I haven't sealed mine yet. Had them for 1 1/2 years and no problem so far.

Ron

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OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

I doubled mine up as well. I keep checking my radius with a radius guage from time to time to make sure.
Just eyeballing it won't give me the true story. There was a very reputable builder around these parts who never took the time to ever check his dishes with a radius guage. One day he finally did...and realized he'd be using a warped dish for probably years It was a minute change that slowly happened.
I just don't want that to happen to me.
-j.

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“If God dwells inside us like some people say, I sure hope He likes enchiladas, because that's what He's getting”
-jack handy

Mine are the same as Ron's, two dishes stuck back to back, one pair 15" the other 25". However, I did seal mine with yacht varnish on all surfaces.

Colin

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

I doubled up also - glued the MDF dish to a 3/4" Baltic Birch substrate for stability, then finished the whole assembly with Shellac...

-Mark

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Pullman, WA

The more I know, the more I know I don't know.

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J. is right on. You should protect it with something. Urethane is good and so is shellac. Relieving the flat side should be required as J. stated. We do this on all of our dishes and it helps tremendously. And just so you know, doubling up material does not always work. When we first started out(before relieving the back), we made some dishes for a friend, and we doubled up 2 3/4" pieces of birch ply that was glued with polyurethane glue and bolted on the edges. The thing was so warped it was unusable. This just goes to show that having more material is not always the answer to a stable dish. It starts with a flat and stable piece of wood. And sometimes you just cannot predict what a piece of wood will do once you relieve one side.
Tracy

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Tracy
http://www.luthiersuppliers.com

I make my dishes out of two layers of 3/4 MDF. I don't releive the bottom but I do drill some holes in it prior to spinning the dish. The lower layer of MDF remains intact and should add all of the support your need, IMHO. I coat my dishes with two coats of water based varnish on all sides and the edges, sanding to 220 between each coat. If you have to remove pressure adhesive paper you will likely still take chunks out of your dish if it is MDF. By going to a one piece paper you will get so many guitars out of taht one peice of paper taht you probably not have to change. If you chip out the dish during paper removal Brad, just take another light pass off of the dish bring ot back to form or use autobody filler and fill the voids, being careful nut to create high spots (which are worse than low spots!)

Shane

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Canada