Spray-On Shoes are nice, but it's not a Growth Industry

Flint sprays shoes onto his feet.
Highly durable polymer.

Logical problems.
He sprays his feet as a child, then he grows into an adult.
His feet should still be "Child Sized" (GOOGLE[ Chinese Foot Binding ]).
In fact they should be even worse if he made the shoes water-impermeable.
If the feet could breathe through the Spray-On Shoes, then this is less a problem.

Think of what happens when you wear wet clothes and wet shoes too long... Trench Foot.

TRENCH FOOT = painful condition of the feet caused by long immersion in cold water or mud and marked by blackening and death of surface tissue.
https://en.wikipedia.org/wiki/Trench_foot

Affected feet may become numb, affected by erythrosis (turning red) or cyanosis (turning blue) as a result of poor vascular supply, and feet may begin to have a decaying odor due to the possibility of the early stages of necrosis setting in. As the condition worsens, feet may also begin to swell. Advanced trench foot often involves blisters and open sores, which lead to fungal infections; this is sometimes called tropical ulcer (jungle rot).

If left untreated, trench foot usually results in gangrene, which can cause the need for amputation. If trench foot is treated properly, complete recovery is normal, though it is marked by severe short-term pain when feeling returns.

Trench foot can be prevented by keeping the feet clean, warm, and dry. It was also discovered in World War I that a key preventive measure was regular foot inspections. Soldiers would be paired and each made responsible for the feet of the other. They would generally apply whale oil to prevent trench foot.