Mold Making Question - How To Make A Negative From A Positive - I'm Confused!

I've made several two piece plaster molds from varies objects I needed to copy. Haven't had any problems until now!
I've got an object which is part of a plastic (fiberglass
acutally) auto body panel. Its called a "dog leg molding." Funny name, I know. Its curved in several directions and has numerous grooves and ridges. I lost the one on the passenger or right side of the car. No one has a replacement. I've checked junk yards, car dealers, etc.
I can easily remove the remaining panel (its about 3 inches wide and 12 inches long) and make a two part plaster cast from the panel. The problem is I need to make a piece the exact opposite from the one I'm casting. In other words all the curves, grooves, and ridges must be a negative of the original positive so that it will fit on the left side of the car instead of the right side.
I've never done anything like this before and I'm confused! In effect, I have to create a negative mold from the dog leg panel. I think I have to start by making a conventional plaster mold and then somehow create a second negative mold from the original positive mold.
If anyone has ever done anything like this before I would appreciate a reply.
Thanks guys!
Holophote
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I think it does get confusing because of the way we humans see things, but actually a mirror image of an object is completely different to its, well- mirror image. What you can do is get an engineering shop to pantograph it into a CAD program and then reverse the axis directions for you, but its not a home workshop project. Easy enough for a computer, but not a by hand job (depending how exact you wanna be).
Chek

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You don't say what make of car, but have you tried scoping out "Hemmings Motor News"?
Over a period of say, a year, if you can't find a part through Hemmings, likely it may not be found, in my experience.
Short of that, I am afraid your only answer is to either make a pattern, using your original part as a reference, either by hand, or by having a pattern-shop do it. I wouldn't be daunted by doing it by hand, as before computers, that's the way it was done.
Art Anderson
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Are we talking a model or a full sized car here? For model work, one can use silicone resin to make the molds, and one can remove the original part (the pattern) relatively easily. For plaster molds, NO undercutting is allowed, one must even have a slight positive clearance angle. So only specially designed patterns generally work with plaster.
Problem is, in translating this to duplicating a full-size part, that the cost of that much RTV gets prohibitively expensive.
If you then MUST use plaster, you must make the mold in many pieces, using clay dams, so that you avoid undercutting, or even parallel sides.
Holophote wrote:

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Can't be done... using the piece from the opposite side as a master will only give you a copy of that piece... best advice is to scratchbuild a replacement part using the opposing piece as a guide for the curves etc. the part can be made from any material then used as a master to remould the part in Fibreglass.
Holophote wrote:

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