Antique Radios found in Auction site or flea market are hardly maintain their original exterior. Especially tuning or volume knobs often would have been lost. Such radios lacking some parts often trade cheaply. So if you don't strictly stick to originality, you can restore complete receiver economically.
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Step 1: Ingredients
Plaｒepair(TM)(Plastic repair kit) is one of resin casting kit, same as "Super-bond" that is popular for dentists.
This time we are going to use these ingredients:
1) Plarepair (TM) powder (black)
This is 100% acrylic powder.
2) Plarepair(TM) liquid
This is Methyl methacrylate
3) "moulage you" (型取り君). mold plastics
It is low temperature deforming material, a kind of polyethylene resin.
3) Another old radio's knob for a prototype
4) A new general-purpose knob
It should be about 5mm smaller than prototype knob, and the hole diameter must be match the shaft of VR or rotary switch.
You can buy Plarepair here. And perhaps there are other products similar to it in your country.
Step 2: Molding
Molding is very easy.
Prepair hot water, dunk moulage in it, and wait a moment.
After it is softened enough to mold, spread it over the prototype and press well. Wait several minutes till it cools down. Then take it off.
Step 3: Casting Resin
Now we have got a mold. Put and drop resin in the mold with the dedicated needle for Plarepair, then the resin spreads on inner surface. With holding against the light keep uniform thickness. After several minutes it consolidate and form a hollow shell.
Step 4: Put Them Together
Put a new knob in the center of hollow, and
fill the gap between new knob and shell with Plarepair powder, then drop Plarepair liquid on it. Be careful not to deviate central axix.
Step 5: Polishing
After taking off the mold, there may be some burrs. Cut them and polish.
Step 6: Weathering
The new cast must look too new and shiny, so it would better be weathered. I splayed gold (or other) paint on cloth, and rub the knob with it.