Bondo on mold

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#1

I open this thread to discuss about a problem i had with the mold i made yesterday. Below you can see a picture of the plug. The plug is made out of steel and MIG welded on areas needed joining. Weld beads can not be made perfectly smooth unless you apply filler to them. So, i decided to apply filler and make a surface as good as i could.

The problem was that after pulling out the mold, some quantity of filler sticked to the mold while some filler remained on the plug.
I must notice that the filler is polyester and i always use epoxy resin. Perhaps polyester filler and epoxy resin are not compatible? Below you can see the spots on the plug where the filler has come out.

And here is the mold and the spots where filler has sticked to it.

#2

Body filler is porous.You need to seal it before you try to take a mold from the part.Something like durabuild primer or 2k paint can be used for this.it can be polished up to a good finish which will make a better mold as well

#3

Plus you need to use more mold release wax or PVA to seal the plug too.

#4

No need for multiple threads on the same thing.

What happened is just a basic failure of tool prep. The plug wasn’t sealed and release coated well enough. Since body filler is porous, it will allow resin to seep in and “grab.”
Did you sand with a fine grade paper? Most body fillers will achieve a smooth dull finish. About 5 coats of release wax should suffice. 5 minutes betweeb wax coats, wait at least 30 minutes after buffing last coat before applying any resin.

#5

I have seen this problem with people WETSANDING filler. Granted wetsanding is key for a lot of smooth finishes…however not direclty to bondo as stated above it is porous and will absorb the water, and thus not create a good bond to the material. Also try to rough up the area in which you will put bondo. Just like anything, paint, filler, glass, it does not like to be attached to surface which is free of any ridges/light scores.

How long are you letting the bondo cure for as well?

#6

Learn how to use various sized fillet shapers and non-drying clay. It will make your life a whole lot easier.

You can make your own fillet tools using dry wall knives and a belt sander. Way cheaper than buying then off the shelf.