Two products that look identical in the bottle behave completely differently on the nail. One dries and the other cures, and the distinction is chemical rather than a matter of quality.
Conventional polish is a solution that evaporates
Traditional lacquer consists of film-forming polymers and pigment dissolved in volatile solvents. Applying it deposits that solution onto the nail.
As the solvent evaporates into the air, the polymers left behind draw together into a continuous film, which is what hardening looks like from the outside.
Nothing is chemically altered in that process, so the same solvents that carried the polish on will dissolve it again, which is precisely how remover works.
Gel polymerizes under light
Gel formulas contain small reactive molecules and a photoinitiator, a compound that fragments when it absorbs light of a particular wavelength.
Those fragments start a chain reaction that links the small molecules into a large cross-linked network, converting a liquid into a solid that was not present before.
The reaction is irreversible, so the resulting film cannot be dissolved back into its components, which is why gel resists chipping and requires an entirely different removal method.
Removal follows from the chemistry
Lacquer is wiped away with a solvent that re-dissolves the film within seconds and leaves the nail plate largely untouched.
Gel must instead be broken down by prolonged solvent soaking that swells and lifts the network, or physically abraded away with a file.
Damage attributed to gel manicures usually originates in removal rather than in wear, because scraping or peeling a bonded film takes layers of nail plate with it.
Adhesion requires surface preparation
A cured gel bonds mechanically to the nail surface, so the surface is typically dehydrated and lightly buffed to give the film something to grip.
Over-buffing thins the nail plate, and because the plate is dead keratin that only grows out, the thinning persists for months regardless of what is applied afterward.
This is the mechanism behind the recommendation to space gel applications, and it concerns the preparation step rather than the gel itself.
The lamp is part of the product
Photoinitiators respond to a specific wavelength range, so a gel cures properly only under a lamp emitting the light its formula was designed around.
An undercured layer remains partly liquid beneath a hardened surface, which causes lifting, sensitivity and contact with uncured components against the skin.
Any persistent redness, separation of the nail from its bed or lasting irritation should be seen by a dermatologist, since those signs have several possible causes.