Need to "Splash" a one-use fender mold.....

[Richard48Y]

I need to "Splash" a one-use fender mold from the red car to repair the body I bought from Joji properly.
Since I have no intention of repainting the red car but also do not want to create an issue for the next owner I have to ask what mold release is the current go to?
Silicone based seems like an obvious NO!
So what is good these days?

[Clifton]

I don't have the answer but I follow a "Fiberglass and Composite" group on FB. A lot of knowledgeable people on it.

[My S1]

Richard,  you'll want to do a throwaway mold out of plaster.  In the past I used Hydrocal B11;  https://www.foamglas.com/en/products/adhevises/hydrocal-b11#  You'll need to buy a bail of hemp and a concrete mixing bucket.  If you decide to go that route I'll be happy to discuss the process with you in detail.

Basically, you will build a perimeter dam around the fender,  build a wooden frame with legs which will attach to the final plaster layer, coat the fender with a parting agent, brush down a 1/4' thick layer of hydrocal and after it mostly cures move onto the bulk of the mold.  The hemp is used to make "bird's nest" wads which will be saturated with plaster and layed down on the first surface coat to create the 1"-2" bulk of the mold.  Before it finally kicks off you attach the legs frame with hemp "ropes".

Plaster will make a very accurate and stable mold from which you can layup a small run of parts.  The main advantage of plaster over say, epoxy and tooling foam , is that it is very inexpensive and you won't feel bad about throwing it away.

[My S1]

Checking my notes,  I used the Hydro Cal B11 for the quick, one shot molds and Ultra Cal 30 for the larger more important molds.  You'll want to research them and choose what works best for you.  They are usually sourced from major building supply outfits.

Attached are pix of a windshield mold and headlamp cover molds to give you an idea.  They are heavy to say the least but can be broken up with a sledgehammer after use.  The thinner the bird's nests layer, the lighter the mold obviously. 

[GavinT]

Agree with My S1.

Plaster/gypsum/cornice adhesive is the safest way to take a form from an existing part and minimise risk to the paintwork.
I've used hessian cloth from the big box store to reinforce the mould . . a lot like working with fibreglass.

But now I'm curious about these hemp "bird's nests".

[Richard48Y]

I like the plaster idea.
I need to do this very soon as I will lose access to the never damaged fender I intend to use.
My main concern of the moment is which release agent to use.
The car I am molding from is going to need paint and I do not want to make that difficult for the next owner.

[GavinT]

For a plaster mould you could do multiple applications of mould release wax. You can't use PVA release agent because the plaster is water based and it'll dissolve the PVA.

Me? . . I'd cover the area with kitchen cling wrap and go from there – fast and easy.
The part won't be as precise as one derived from a traditional fibreglass mould but well within usable parameters for a panel that'll require some finishing in any case.

[My S1]

 2-3 coats of Partall #2 paste wax will do the job on a painted fender BUT you have to be weary of pulling the mold off after it is cast.  There must be no undercuts.  In the case of the headlamp bucket, you will need a seperate mold .   You have to study the surface closely and first figure out how much you want to capture then how you want to pull the plaster off.  You also naturally have to consider how you will pull the part from the plaster mold.  Take allot of time in planning.  The actual plaster work is a breeze.  Hessian cloth (burlap) is another alternative to hemp clumps.  3-4 layers saturated in the plaster will give you a relatively thin mold but definitely strong enough to pull a fender or two.

[My S1]

Richard,  you will need to establish a pull vector.  A simple way to get an idea of the proper direction of pull is to get a powerful flashlight, turn off all the lights in the garage and point the torch at the fender.  Move the light around and scratch your head until you get an idea of the of what's going on.  Where the light fades out will give you a rough idea of where the edge (singular) of the perimeter walls should be.  Crude but it will get you started.  Obviously, you don't want the edges of the mold to undercut the fender surface.  If you can't capture the full area that you want, you'll need multiple mold sections.  Cast one, then the other adjacent ones sharing the wall of the first.

If I were going to do this on my fender, I might use that 3/4"(?) pink firm insulation foam for the walls.  You can use double face tape on the edges of the walls onto the fender then duct tape the vertical edges to the fender on both sides.  Wax the hell out of it and have fun with the plaster.  Any number of materials can be used for the walls.  I naturally use Chavant automotive modelling clay but any number of materials, cardboard, styrene sheet, you make the call, can be used.

[Richard48Y]

Fortunately this is only a section of the fender.
It will however require the underside fender lip.
So I always expected to do a two-piece mold.