I've been working on a Halloween prop that will need many, many nuts & bolts as details, actually more than I want to count once finished. Because of that and my usual empty wallet, the bolts I'm using are fake. What you see above are just simple, homemade, lightweight, glued on applique copies of the real thing.
Here's a cheap, easy and I think, fun way to mold and cast small details with stuff you might already have around the house or find in any craft and hardware store. This method is not for those hoping to make perfect copies, but for those okay with slightly flawed details on a prop. For me it works, the aged and worn feel is perfect for Halloween. It's also a painless rookie introduction to molding and casting. Know that there will be trial and error doing this. It's just one of those things. You'll make a few screwed up ones. The good news is, it's so simple and easy that the results of your efforts are seen in minutes and just as quickly re-done if needed. For this "how to" I'm making the nuts & bolts I need but this trick can be used for creating many different details for other projects or props.
-Fix-It-All patching compound or similar.
-Non air-hardening Polymer or Plasticine clay.
-Disposable cup and spoon for mixing.
-Needle or pin.
-Fine grit sandpaper
-bolt or something to mold
Plaster or drywall mud can also used but I prefer Fix-It-All, it's a bit stronger than the other two once dry. The clay may stain certain objects so pick your molding subject carefully. The mold impression will usually be ruined after the casting is removed. So assume one copy per mold but you'll be able to re-use the clay after. If you need many casts, make a few molds while your at it. For outdoor props there is an exterior grade patching compound available or protect with a water sealing type clear coat (which you can do before or after painting). If you haven't noticed yet, I tend to over explain things sometimes.
Make a clay ball about 1" wider in diameter than the size of the nut & bolt. Next, firmly but slowly, push it into the clay straight down. Trying not to lean, turn or twist it. Then carefully pull it out the exact same way. There may be some slight distortion made to the impression while removing, try mushing the clay a bit until it looks right. Now you have your mold.
Use a dust mask here. In the cup, make a thin mix of Fix-All compound. Gravy thickness works for me, about a shot glass amount of water and one heaping spoonful of powder. This is more than enough for one bolt. It hardens very fast, so only mix what your going to use at a time. Pour slowly into the mold trying to avoid bubbles. Poke any with a pin or needle. Aesthetically, a few bubbles are not a problem as they usually rise to the top on the unseen back of your casting, but if too many they will create a honeycomb that can easily crumble and break away once dry.
Almost immediately you'll notice the mix will get heavier as it sets up and sink, forcing excess water it to the top. Dip a paper towel edge in to soak up the water, then add more mix. Repeat this as needed until you have a full level of solid mix. That's it, let it set up and harden at least 20 - 30 minutes. Larger or thicker molds will take longer.
The fun part. Carefully pull the clay away and remove the casting. I recommend you set them aside for another hour to dry before sanding and painting. Just a side note, in the picture below you can see the yellow molds came out very clean with almost no residue on the clay, the red did not. This is due to my impatience popping out the casting when it had not fully set up. Not a problem though if it happens, the hardened residue can easily be scraped off to re-use the clay.
Step 5 :
Just some casting clean up tips. A fully cured one is best to work with and clean, it will turn a tea stained color especially darker at the edges when ready. Sand or trim edges with a fine grit sand paper but go slow, it's easy to remove too much working fast. Any unwanted gaps or holes can be filled by rubbing in some dry compound and dabbed with a wet finger or paintbrush.
Painting. Any paint or stain works great on Fix-All. It's very porous so a few coats or primer will be needed. For mine I used some brush-on chrome model paint and finished with a watered India ink wash, letting it get into all the recessed areas to create more depth. Depending what your attaching it too, Fix-It-All works well with almost any glue. As I mentioned earlier, if using for outdoor props, you should protect with a varnish or use exterior grade compound.
Here are some quick castings I made of random things around my studio.