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I just had a disappointing experience using a bondo-like product to attempt to fill gaps in a mold I am making. I am building up MDF and routing each layer. I had the latest layer glued out of register so the router did not contact a few spots. I attempted to fill the gaps with the bondo. When applying the wet resin it did not seem to wet the MDF, but I just pushed it up tight and hoped it would work. Two problems occurred. First, upon routing the bondo it produced shavings perhaps molecule thick that went everywhere and adhered by static--a mess. Second, the patches did not totally adhere and pulled out in many locations.

So, first is a warning to those contemplating this. Second is a request for advice for a filler for this situation, if I screw up the register of the layers again. Third is a request for what, if any, wood products to bondo adheres to well. Years ago I used it on particle board to build outside molds using the inside mold as the casting form, and it seemed to work fine. I'll probably be testing some samples of various wood products, but any experience out there would be welcome.

I suggest drilling though the layers to keep them registered with dowels or screws. Sorry, no advice on the Bondo.

Ditto --- the template should have dowel locators (But I am not clear on what you are actually doing) -- at least that is how I did it before we had CNC machines. Been using Bondo for thiry years+ great product, never had a problem with it adhering to "anything and everything" - but scuffing the MDF surface or drizzling Thin CA along the patch won't hurt. We use the Gold Bondo which is the same as the household wood repair version. I just view dust shavings and static cling as "stuff happens" and deal with it appropriately.

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Ken Cierp

http://www.kennethmichaelguitars.com/

As you add each layer of MDF cut it slightly oversize (band saw or jigsaw) then screw to the previous layer and route with a bearing guided flush cut router bit.

Bob

Small areas can be filled with baking soda and thin CA -- it forms a fairly hard substance that is very easily sanded.

Another one of my posts disappeared.

I was previously using screws, but I got overconfident/lazy.

The shavings are little ribbons 3/4" x 1/8" by molecule thick. They adhere and do not brush up with a broom easily. Also, some of them resist vacuum. So, they are a pain I hope to avoid in the future.

I've had to add material to a spot or two on a form. I just use some left over back and side wood. I glue it in with titebond then the next day go back to shaping it. I works fine with router, file, or sanding.

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Joe Beaver
Maker of Sawdust

When using bondo to fill gaps and divots you can estimate the proper amount, place wax paper over top of it and smooth it in place with a wood block or other tool. Peel off the wax paper after it hardens. A little bit of sanding may be all you will need to finish it off.

Some great filler suggestions here. Thanks!


Ken Jones
Mountain Song Guitars

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Mountain Song Guitars www.mountainsongguitars.com