They are sibling resin chemistries with opposite temperaments, and modern flooring uses each where its temperament fits:

Polyurethane's virtues: flexibility — cured PU films flex with substrate movement and absorb impacts that chip rigid epoxy; abrasion resistance — PU wear layers outlast epoxy under scuffing, castors, and foot traffic, which is why commercial wear coats are PU; UV stability — aliphatic PU does not yellow or chalk in daylight, where standard epoxy visibly ambers near doors and windows; and thermal shock tolerance — PU screeds survive hot wash-downs and cold-store cycling that crack epoxy.

Epoxy's virtues in the comparison: higher build per coat, harder point-load resistance, stronger adhesion to marginal concrete, generally cheaper per litre, and better tolerance of application-day imperfection.

How the trade resolves it: the classic system is epoxy body + polyurethane top coat — epoxy primers and build coats do the bonding and filling; a PU wearing surface takes the traffic, the sunlight, and the scratches. Standalone PU earns the whole job on timber floors (flexibility over moving boards), thin-build commercial refurbishments, exterior-exposed slabs (UV), and the food-industry PU screed tier.

The buying shorthand: epoxy is the structure, polyurethane is the skin — specify by which problem the floor actually has, and don't pay PU prices for a cellar the sun never finds.

PU leads wherever its three signatures — flexibility, abrasion life, UV stability — match the duty:

1. As the top coat over epoxy systems: garages and showrooms wanting gloss that stays un-scratched and un-yellowed; flake floors sealed under PU keep their colour at the roller door where epoxy-only systems amber. This is PU's volume role.
2. Commercial and retail wear floors: shops, cafés, offices, and galleries — thin-build PU systems over concrete take castors, heels, and cleaning machines with the longest re-coat intervals in the resin family.
3. Timber and moving substrates: PU's flex makes it the resin for wooden floors, mezzanines, and boards — where epoxy's rigidity cracks along every joint.
4. Exterior and daylight-flooded slabs: balconies (with appropriate systems), covered externals, and glass-walled spaces — aliphatic PU holds colour where sunlight retires epoxy.
5. Food, beverage, and thermal-cycling industry: heavy-duty PU concrete screeds (urethane-cement hybrids) are the standard for breweries, dairies, and commercial kitchens — steam-cleanable, thermal-shock-proof, and tolerant of the organic acids that attack epoxy.
6. Car parks: PU deck systems flex over dynamic concrete and waterproof it — a specification category of its own.

Where PU concedes: bare-concrete penetration priming (epoxy bonds better), maximum-build repairs (epoxy mortars), and budget-led single-material jobs where epoxy's price wins. The pattern across every case: PU is chosen for the surface's working life, epoxy for the bond and the build — and the best floors buy both.

The UV story splits polyurethane into two chemistries, and the label matters more than the brand:

Aromatic polyurethanes (the cheaper backbone) react to UV — films yellow, amber, and eventually chalk with daylight exposure, much like standard epoxy. Indoors away from glazing they perform fine and price well; specified in a sunlit space, they quietly become the discolouration they were bought to avoid.

Aliphatic polyurethanes are the UV-stable tier: colour-fast under direct sunlight, gloss-retentive for years, and the reason 'PU top coat' earned its reputation. Exterior systems, showroom floors, light-coloured and clear finishes, and anything within daylight's reach should specify aliphatic explicitly — the datasheet will say so (and the price reflects the roughly 30–60% chemistry premium).

Practical selection: clear coats over decorative flake, metallic, and timber floors are the highest-stakes case — yellowing shows worst in clears over light substrates, so aliphatic is non-negotiable there. Solid mid-greys hide early ambering longer, buying tolerance where budget forces aromatic. And note UV stability protects colour, not everything: even aliphatic films wear at door thresholds where sunlight teams with grit — entrance mats remain undefeated.

One adjacent chemistry worth knowing: polyaspartics — fast-cure aliphatic PU cousins that cure in hours rather than days, enabling one-day garage installs at premium prices. Same UV virtue, compressed schedule, less forgiving application window: professional-leaning, and increasingly the top-coat upsell.

Application follows the resin-floor playbook with PU-specific disciplines:

1. Substrate prep as for epoxy: diamond-ground, clean, dry concrete (or sanded, sound timber); PU forgives nothing epoxy wouldn't. Most systems start with an epoxy or PU primer matched to the substrate.
2. Mixing: two-pack PUs mix rigorously to ratio — PU hardeners are moisture-sensitive in the can (once opened, use or lose), and stragglers of unmixed hardener leave permanently soft patches. One-pack moisture-cure PUs skip mixing but demand ambient humidity in range.
3. Application: short-pile roller in thin, even passes — PU builds thinner per coat than epoxy by design; two to three coats typical. Maintain a wet edge (PU shows roller overlap marks in gloss more than epoxy) and work to the recoat window: most PUs recoat within 12–36 hours or need abrading.
4. The humidity rule: PU cure involves or tolerates moisture chemistry, so datasheet humidity limits are real — too dry stalls moisture-cure products; condensation-point conditions blush any of them. Measure, don't guess, on marginal days.
5. Additives and textures: anti-slip grit goes in the final coat exactly as with epoxy; matt and satin versions need thorough stirring to keep flatting agents suspended.
6. Cure: foot traffic typically 24 hours, full hardness and chemical resistance 5–7 days; polyaspartic variants compress this to same-day.

The craft note that separates finishes: PU rewards thin coats and patience — the temptation to build it like epoxy produces solvent-trapped soft films, while three disciplined thin coats produce the furniture-grade surface the chemistry promises.

Durability is PU's home ground — and maintenance is mostly about not sabotaging it:

The wear picture: PU wearing surfaces in commercial service run 5–10 years between re-coats under traffic that dulls epoxy in 2–4; taber-abrasion numbers (the datasheet wear metric) routinely show PU losing half as much material as comparable epoxy. Flexibility means impacts dent-and-recover rather than chip, and hairline substrate movement doesn't telegraph as cracking. Chemical resistance covers oils, fuels, and most cleaners; strong solvents and concentrated acids stay in the epoxy-novolac specialist tier.

Routine care mirrors resin-floor practice: grit control first (mats, regular dust-mopping — abrasive dust is the only thing that meaningfully wears the film), washing with pH-neutral cleaner and soft pads or auto-scrubber, prompt spill wipes as good habit, and no abrasive powders, no steel wool, no acid descalers.

PU-specific notes: gloss PU shows fine scratch swirl under harsh cleaning sooner than its wear life implies — satin finishes hide honest use best in busy spaces; burnishing/polishing machines with aggressive pads belong to other floor types; and felt glides under furniture remain the cheapest life-extender in the book.

The renewal cycle that makes PU effectively permanent: when traffic lanes dull or scratch density accumulates, a screen-and-recoat — light abrasion plus one fresh PU coat — restores the floor in a day at a fraction of system cost. Floors managed on that cycle (commercial practice: every 5–8 years) never reach failure at all; the substrate system underneath simply keeps working under successive skins.

Yes — timber is polyurethane's oldest flooring franchise: 'floor varnish' in the modern sense is PU, and the professional wood-floor trade runs almost entirely on it.

Why PU suits wood specifically: boards move — seasonally, structurally, underfoot — and PU's flexible film moves with them where brittle finishes crack at every joint line. Its abrasion resistance handles grit-on-timber traffic; its build fills grain into a cleanable surface; and water-based aliphatic PUs stay glass-clear over pale oak where oil-based amber warmly over darker species (a colour choice, not a defect — sample both).

The product tiers: one-pack water-based PU (low odour, fast recoat, the domestic standard), one-pack oil-modified (warm tone, slower, traditional), and two-pack PU lacquers (the commercial-wear tier for shops, halls, and stairs — meaningfully harder, at professional application discipline). Sheen runs gloss to ultra-matt; matt hides wear and suits current taste.

Stairs deserve their specific notes: treads concentrate wear like nothing else in a house — use the two-pack or heavy-duty tier, three coats minimum with sanding between, and consider anti-slip additive or inserted nosing strips on the going, because glossy PU on a smooth tread in socks is a proven accident format. Refinish treads at first wear-through rather than after, and stagger the job (every other tread) if the staircase must stay in service.

Application on timber: sand to fresh wood (or abrade sound existing finish), vacuum-and-tack dust obsessively (dust nibs are the visible failure of amateur PU), thin even coats along the grain, light inter-coat abrasion, and full cure — days, not hours — before rugs return. Done so, a PU timber floor's maintenance is the screen-and-recoat cycle measured in decades.

For a garage specifically, the honest engineering answer is: buy both, in their right places — epoxy body, PU (or polyaspartic) top coat — and whether that premium over epoxy-only pays depends on three things the owner already knows:

1. Daylight: a garage whose door stands open, or with windows, will amber an epoxy-only floor near the light within a few years — cosmetic, but exactly what a proud floor was bought to avoid. Aliphatic PU top coat solves it permanently. Windowless basement garages can skip this argument entirely.
2. Use intensity: a parking-only garage barely tests a quality epoxy's wear life; a workshop garage — trolley jacks, foot traffic, dragged benches, dropped tools — dulls and scratches epoxy's surface years before its bond fails, and the PU skin's abrasion life directly extends the years-between-looking-tired.
3. Finish investment: decorative flake and metallic floors put their beauty in a clear coat — the one film where yellowing and scratch-swirl show most, making aliphatic PU (or polyaspartic) clear coats close to mandatory on decorative systems. Plain grey utilitarian floors have the least to protect.

The premium in numbers: a PU top coat typically adds 20–35% to a garage system's material cost — against which it roughly doubles the surface's cosmetic lifespan and removes the yellowing risk. Skip it defensibly on: dark solid colours, sunless garages, strict budgets. Specify it without hesitation on: flake and clear-coated floors, sunlit doors, and any garage doubling as a workshop or showpiece.

And if the schedule matters more than the invoice — the polyaspartic route delivers the PU virtues with one-day installation, the premium's premium.