Taking Impressions of Moulding

Advice on making a reverse impression of a moulding you want to replicate. November 8, 2008

Question
I'm looking for a method of matching trim pieces on an existing desk. I have tried the needle gauge and the trim is just too complex and small. Does anyone know of a modeling clay or something similar that is firm enough to hold the shape when pulled away? I have tried painter's putty but this and other similar products are too sticky and deform as pulled away.

Forum Responses
(Cabinetmaking Forum)
From contributor M:
There was an article some years back in Fine Homebuilding where the author used Bondo to make “negatives” of existing moldings, which he then used to have knives made. He taped off a section (maybe 1-1 ½” depending on the size molding), mixed the Bondo and smeared it on, added pieces of wood to strengthen it, let it set up and then popped it off. I think that he used cooking spray for a release agent so the Bondo would not adhere to the finished surface.

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From contributor A:
Lay the desk in a position that will allow the Bondo to sit on top of the moulding. Mask off around the area to be copied with blue masking tape. You can use wax paper or plastic wrap to protect the piece of furniture. Take Minwax paste wax or butcher's wax and liberally spread it over any surface the Bondo will come in contact with. Make a little form out of cardboard and tape it together. Maybe 3" long and as wide as the moulding. Pour enough Bondo to cover at least 1/2" of the entire moulding. Embed a piece of scrap wood on top of the Bondo. Add a little more Bondo around the scrap to make sure it's a well bonded mass.

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From contributor F:
I will chime in about reverse sanding blocks. Depending on the particular profile and quantity needed, you can sometimes apply PSA sandpaper to the desired molding profile to be duplicated. I will usually try to trace the profile from an exposed molding end onto the sanding block blank and then rough out the reverse profile with the table saw, etc. Then you can rub the reverse sanding block blank on the sandpaper covered molding profile until the shape has been completely transferred, checking progress by means of pencil lines that disappear when all surfaces have been fully abraded.

I have used lots of different wood species and also Styrofoam for reverse sanding blocks. The absolute best is balsa wood. The balsa sands rapidly and as a bonus, its soft and spongy nature makes it behave similar to a cork backing for increased sandpaper life.

Adding the sandpaper to the profile being copied changes it by the thickness of the sandpaper. This difference is canceled when the same thickness of sandpaper in used on the reverse sanding block to make/sand molding. Of course this technique is only useful for a small amount of molding or to sand moldings made by conventional means.

To sand a moulding from scratch is much faster if the molding blank is roughed out on a table saw with multiple passes at multiple blade elevations.

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From contributor B:
Skip the Bondo - it sounds like nothing but trouble. There is a Master mold kit that looks very good, though I've used silputty very successfully. Sold by Rockler, I think, or similar. It also has green and gray components.

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The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
Bondo works great. Just apply clear tape to the wood surface, apply wax on top of that and then the bondo. As soon as the bondo hardens and is still hot just slowly remove from molding. Apply temporary adhesive to the block and go to town. I completed one in about five minutes.