Official Luthiers Forum!

If you use an MDF mold what do you use to protect it from wet sides?
Jason

Mine aren't MDF and I haven't put wet sides in them, but I suspect that you could use some poly varnish on them and perhaps cover the insides with some packing tape.

just a spit coat or two of shellac is good enough protection for the mold. the sides should be completely dry when you put them in the mold though. you should cook all of the moisture out when bending, otherwise you will end up with cupping and all sorts of other nonsense.

_________________
sweat the small stuff.

Yup, what Heath said.

_________________
My Facebook Guitar Page

"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson

Thanks guys.
Jason

Is MDF suitable for molds? I am going to remake my mold and am out of Plywood, but have a bunch of MDF.

Thanks,

John

MDF is suitable for molds. Plywood works equally well and is cheaper then mdf. But if you have mdf on hand use what you got.

If you already have the MDF, use it up first. I have found that it is way easier to shape and you can get a much smoother finish on the edges than you would with ply. Ply is great for structural stuff...maybe make the face part of forms from MDF and the rest from ply?

It has been my experience that MDF makes at best an iffy mold. MDF is subject to moisture . If you are making a prototype it may be ok but it won't last long term. You also need to seal it well with a water proof sealer . polyurethane would be my first choice. Shellac isn't water proof.

_________________
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

That is really good to know. I made up some radius dishes out of MDF and sealed them with Zinser Shellac. Being in Florida, I was attempting to keep the moisture out. I guess I need to recoat them.

No, shellac definitely is not waterproof (liquid form), but I do believe once cured, it is essentially water vapor (humidity) proof. I'm sure a web search could verify this information - sorry I don't have the reference any more.

Would it be a good idea to spray a marine grade resin, or a fiberglass 'gel coat', on the surfaces of the forms? It may take a few coats to compensate for 'soak in' so you would need to take the thickness of the resin into account. Just thinking out loud.

-Matthew

I'd have to agree with Todd & John Hall.... plywood over MDF. But I don't think high grade is really necessary or adds much. I use 3/4" ply "sheathing" (making one up this morning actually). At $10 for a 2' x 4' piece (from which I get 6 "half-mold" pieces that I finger-joint together), it's hard to get a more cost effective deal!

_________________
Dave
Milton, ON

There is a BIG difference in plywoods. Aside from the wood species , glue , grade etc stability is key. Baltic and finnish is higher grade , more stable and machines well. Sheeting grade will warp , it also has voids. If you are making guitars seriously , you need to look at tools as investments not cost.
Shellac is hydroscopic , it will allow moisture to pass . Even in its cured state. Utathane and Polyurathane are waterproof. Marine finish is expensive but will work but read the label. Some marine finishes don't really cure and remain tackey

_________________
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

I got some poplar plywood from the depot and it isn't bad plywood for molds and forms at around $40 a 4 X 8 sheet. I agree with staying away from mdf except where it's a good choice and I think not for molds.

Ok since the topic is MDF and it looks like MDF for molds is OK but not the best. I have a 24"x24" section and a 48"x24" section of MDF that I have not figured out what to do with (I had been thinking I would use it to make a new mold). Would this be problematic as gobar deck material if I treat it right?

Most of my molds and jigs are going to be housed in a Florida garage where humidity is high. I am working on controlling it but I am going to have to do it in baby steps. I might be able to some day slip a window AC unit in, but the window is right in the front of the house.

Ya know, I can't really agree with the "mdf is second to plywood" position. First the moisture issue is a none issue in a shop where relative humidity is controlled at 45% or so. True, they are heavier, but in some instances that may be better (like when I use them to place on my rim sander). MDF does not hold screws well but does glue well, so for attaching hindges and latches I just glue on a piece of wood. And has been stated earlier, mdf will finish smoother and is a bit easier to shape than plywood. I won't go as far as to say that mdf molds are better than plywood, but I also would not agree that they are inferior, just different!

I use mdf for molds and bending forms without issue and will continue until I have a problem.

Good luck!

Shane

_________________
Canada

The only thing I can think that would be a setback with using MDF is what Shane mentioned about screws.

If you're planning on using latches or anything that requires screws in MDF, you would first need to do as Shane said and glue wood to the MDF to hold the screws because the MDF will not.

_________________
Thanks,
Blain

http://www.ullrichguitar.com

"89.67% of all statistics are made up on the spot."

Although I have a few spare collars that are plywood, all of my molds are MDF and I've had them for years. I seal them with a penetrating epoxy used to fix boat hulls that have delaminated. The MDF machines better than any plywood I've used, and despite the suggestion, baltic birch can and does warp. It depends upon how the supplier stores it.

Yeah, I can see where the humidity will be a big issue for your setup. In the Portland Oregon area we have lots of wet, but the lower temps keep the humidity lighter. I have used MDF for different projects before and seldom have issues with it getting 'soggy'.

You are right about reading the bottles on resins and epoxies...some are not 'waxed' and will remain tacky (so the next layer/application will stick easier I think). Check out Tap Plastics for more info on that (and they have good info on composites for double tops, etc).
I would think that bondo would be another option for building custom shapes/forms and cleaning up the edges of plywood forms that have chipped or have gaps/voids.

Just my 2 cents worth.

MDF will warp before Baltic birch. There are more than one grade of baltic birch. Pattern grade is a commercial product that is used in the pattern making industry. The strength factor if you use spreaders makes ply better than MDF , there are more advantages to using Baltic birch than there are using MDF but lets be honest , cardboard can make a mold.

_________________
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

Pete Licis

D. Berkowitz

Wow, I'm in a time machine! Hey guys!

Mine are plywood floor decking called advantech. Leftovers from home construction. Why not? Heavy, you bet.

With all the laminations in most molds, it all works. MDF, particle board, plywood, OSB which Advantech is a high quality version.

_________________
http://www.dickeyguitars.com

I am with David. I use MDF for some of my molds and they have worked great. Instead of screws I have glued, pegged and pinned them and they work fine.

In fact I have a number of bending molds and lamination molds that use it too.

I have never had any issues.

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com