The caveat is that this process, assuming the average bicycle tyre costs anywhere from $20 to $80 - or more, it can be retreaded for about $2 a tyre.
On very smooth roads, the expected mileage could reasonably be expected to be about 2000Km - 4000Km per retread.
It's very easy to do, it can be done with the wheels in the bike, and one ought to allow about 24 hours for the process to finish.
Everything in life is a compromise - this process and materials are no exception.
We use NEUTRAL cure silicon sealant, to build up the worn tyre with.
This has some interesting properties.
After the tyre has been sanded, when applied properly, it is extremely adhesive. It STICKS to the tyre.
It has a low shear strength compared to normal organic carbon based rubbers - meaning that building it up into a "thick retread" is not especially worthwhile, unless you and the bike are light and the tyres are fairly wide.
So the recommended build up is for a shallow dome, at best about no more than 2 or 3mm above the original tyre profile.
The optimum conditions of longevity are for an light to average weight rider, on a bicycle with nice wide tires, on smooth roads - doing the daily commute and travelling etc.
The heavier the rider, the more sharp sharp stones and very coarse gravel, and the more aggressive the riding style, the shorter the retread will last.
And the truly interesting bit.
Silicon rubber is extremely hydrophobic - it chemically repels liquid water.
This means that while you end up with a NICE long lasting retread that works very well on smooth dry roads and paths, when those roads are wet, the effect is that the silicon, while it deforms over the surface of the road, it molecularly repels the water on the surface of the road.
The effect is much like trying to walk through a kitchen with cooking oil spilled on the floor.
You can ride across wet patches, provided the road is level, you bike is within a few degrees of being upright, your not cornering, braking or accelerating, and you do it very very carefully. But for all intents and purposes, regular riding on these retreads in the wet, is impractical and unsafe.
The good thing is that there are some 500 million plus bicycles, and for most people, in most parts of the world, most of the time, most of their riding is done and can be done in the dry. There are also many people who tend to have seasonal weather, who can swap over to regular or wet weather tyres for the season, or can travel by other means, or can wait it out for the occasional shower to pass.
That makes this retreading system, suitable for about 95% of the people, about 95% of the time.
I myself live in an area, where in an average 30 day period, there may be about 630 hours of dry good riding weather, with perhaps 10 hours of rain. That is about a 98.5% riding time.
The back stop is that while the silicon could be compounded with silica flour (ground sand) or carbon black, to make it grip better in the wet, compounding it reduces the the tensile strength of the silicon rubber making it unsuitable for retreading the tyre.
There is commercial potential there, but my knowledge of compounding silicon adhesives, leads me to believe that the benefits of the adhesive / sealant - and the polymerisation of silicones, cannot be reconciled. Either one has a cheap, easy to apply and durable retreading system, or one may be able to achieve "carbon rubber" like wet weather characteristics - but the silicon rubber ends up like soft cheese.
One may be able to mix in SOME silica / silicon flour / ground sand - to improve the wet weather grip, but I think it's a degree of proportion. The addition of SOME may lead to a big improvement in wet weather adhesion, but too much, may make the silicon soft and worthless.
As it stands, you could probably retread your tyres 10 or 20 times or even more, and this is a substantial saving on resources, such as time, money, and mechanical efforts.
The tyres in terms of longevity, will last longer if you keep your bike out of the strong sun, where ever possible. UV protection with a wipe over with some ArmorAll is a good idea.
The safety issues.
You now have a way to have near infinite retreads with your bike tyres, with the only real caveat that you CAN NOT ride it on wet surfaces.
One must also be mindful that tyres were intended to provide good service, not indefinite service. Retreading them annually for 30 years could be regarded as having gotten quite a great deal of extra service from them... Though 30 years on the same tyres might be pushing it a bit, 10 or 12 years of regular retreads, is entirely reasonable.
Tyres ought to be thoroughly inspected on a periodic basis - weekly seems to be fine, for damage, such as tears, cuts, bulges, cracking, fraying, separation of the carcass and tread etc., and replace them if necessary.
And don't ride on them in the wet.
This idea "may" have real commercial potential - but I decided that it's best provided in the public domain, and because it's real basic corner store tech - where anyone and everyone can do it, it's SO easy and low cost - in regards to our current "opportunities" in the global sense, it's necessary.
More people on bicycles more often = good. Better bicycles lasting way longer = good. One set of bicycles tyres lasting as many as 10 or 20 sets of bike tyres = very good.
These are the issues we need to consider.
So retread with gusto and enjoy the benefits of corner store tech!
Step 1: Prepare Tyre.
Plywood is fine, so is plastic, wood, metal etc.
You can retread to make the tyre the original size, or add a little extra to get more distance between retreads.
2 or so mm more is good, 3mm or so is getting excessive, unless you weigh hardly anything and your bike has large diameter road tyres then 4mm probably would be the total thickness limit.
My tyres are 35mm in diameter.