The 30-second answer
If you are deciding between epoxy and polyester for a casting, art, or table project, epoxy is the right default and polyester is the cost-driven exception. Epoxy wins on almost every metric that decides how a finished piece looks and lasts: it shrinks only about 1-3% as it cures versus roughly 7% for polyester, it bonds far harder (around 2000 psi against under 500 psi), it cures near-odorless instead of filling the room with styrene fumes, and some formulas are BPA-free and safer for food-contact tops. Polyester’s only genuine advantages are a lower price per gallon and a faster gel-to-demold cycle.
Put plainly: choose epoxy for river tables, clear castings, jewelry, and coasters where clarity, low shrinkage, and durability decide the result. Choose polyester mainly for fiberglass lamination, fast turnaround embedments, and budget bulk casting where some shrinkage and a styrene smell are acceptable trade-offs. The comparison table and the per-product specs below break down the exact numbers behind each call.
How each resin actually cures
The two resins are fundamentally different chemistries, and that difference drives almost everything else.
Epoxy is a two-part system: a resin and a matched hardener that you combine at a fixed ratio. Deep-pour formulas like TotalBoat Fathom mix at 2A:1B by volume (100A:44B by weight), while table-top coating epoxies like Pro Marine run a simpler 1:1 by volume. The two parts react with each other to cure, so the ratio is non-negotiable - get it wrong and the resin stays tacky or never hardens. There is no separate catalyst to dial in; the hardener is the second half of the system.
Polyester is a single resin that you kick off with a small percentage of MEKP (methyl ethyl ketone peroxide) catalyst. Castin’Craft and Fiberglass Warehouse both ship MEKP in or alongside the kit, and the dose is small and fussy: roughly 1.25-2%, and never over about 3%. Too little catalyst and the casting stays soft; too much and it overheats and turns brittle. That catalyzed reaction is fast - polyester typically gels in 10-20 minutes and demolds in hours, where epoxy takes 24-72 hours to demold and days to fully cure. Speed is polyester’s headline advantage and the main reason fiberglass shops favor it.
Shrinkage and dimensional stability
This is the single biggest reason crafters pick epoxy for castings. As it cures, epoxy shrinks only about 1-3% by volume. Polyester shrinks roughly 7% - more than double. In a thin lamination that barely matters, but in a casting it shows up as parts pulling away from inclusions, sink marks over thick spots, and internal stress that can crack a large block. If you embed a coin, a flower, or a piece of wood and the resin shrinks 7% around it, the bond at that interface is fighting the shrink the whole way. Epoxy’s low shrinkage is why river tables and deep clear castings hold their shape against the wood instead of cupping or separating. The specs below list shrinkage class for each product so you can see where every resin lands.
Strength, adhesion, and brittleness
Epoxy is both stronger and tougher. Its bond strength runs around 2000 psi, while polyester comes in under about 500 psi - which is why epoxy is treated as a structural adhesive and polyester is not. Epoxy also stretches further before it fails: tensile elongation at failure is roughly 3.5-4.5% for epoxy versus about 1-2% for polyester, so polyester is noticeably more brittle and more prone to cracking under impact or thermal cycling.
There is a well-known data point from WEST SYSTEM’s repair testing that captures the gap. On a 26,198 psi control laminate, an epoxy repair restored 21,404 psi - about 81.7% of original strength - while a polyester repair recovered only 18,460 psi, or 70.5%. Epoxy’s secondary bond to a cured surface is simply better, which matters any time you are layering, repairing, or building up a piece in stages.
Depth and pour limits
Pour depth is where the product choices diverge most, so match the resin to the job. Table-top epoxies are flood coats: Pro Marine tops out around a 1/8 inch flood coat and 1/4 inch maximum per layer, which is perfect for bar tops, counters, and art panels but useless for deep castings. Deep-pour epoxy is the opposite tool - TotalBoat Fathom goes up to 2 inches deep for river tables and 3 inches for larger castings in a single pour, because its long 4-6 hour working window and lower exotherm let a thick mass shed heat without boiling.
Polyester casting resin sits in between but is capped low: both Castin’Craft and Fiberglass Warehouse are limited to about 1/4 inch per layer, so any real depth has to be built up in multiple pours. Fiberglass Warehouse cures with a surface tack so successive layers bond without sanding, which softens the inconvenience, but you are still pouring in stages. For a genuine deep pour, deep-pour epoxy is in a class polyester cannot reach.
Clarity and yellowing over time
Both resins can be crystal clear when fresh - polyester casting resins are prized for embedding coins, shells, flowers, and insects in glass-clear blocks. The difference is how they age. UV-stabilized art epoxy resists yellowing well, while polyester yellows more readily under heat and UV exposure. If a piece will sit in sunlight or under hot lights, epoxy holds its color longer. Note too that even good outdoor epoxy needs a UV-blocking topcoat or marine varnish for long-term sun resistance - epoxy resists yellowing better than polyester, but no clear casting resin is fully UV-proof on its own.
Odor, styrene, and indoor safety
This is a real, not cosmetic, difference. Polyester resin can be up to roughly a third styrene by weight, and it off-gasses a strong styrene odor that irritates the respiratory tract. Epoxy is near-odorless and low-VOC. For indoor work that gap decides which resin is practical at all.
As a conservative safety note: with polyester you genuinely need cross-ventilation and a NIOSH-approved organic-vapor respirator with activated-charcoal cartridges - a dust mask does nothing against styrene vapor. Standard polyester also carries a California Prop 65 warning, and Fiberglass Warehouse offers a low-styrene variant specifically to cut odor for indoor use. With either resin, wear nitrile gloves: uncured resin is a skin and eye irritant and a sensitizer, and repeated unprotected contact can cause lasting sensitization. A simple rule - if you can smell the resin, your ventilation is not adequate. Always follow the manufacturer’s SDS for the specific product you buy.
Food safety and toxicity
If anything will touch food, drink, or skin regularly, this narrows your options fast. Polyester casting resins are not certified food-safe and off-gas styrene during cure, so treat any polyester piece as decorative only. Among epoxies, food-contact safety is formula-specific, not automatic: TotalBoat Fathom is BPA-free and marketed safer for food-contact surfaces once fully cured, while Pro Marine’s table-top epoxy is not certified food-safe. For coasters that hold a cold glass, a fully cured low-shrinkage epoxy is the sensible pick; for anything that contacts hot food or acidic liquids, verify the specific product’s food-contact claim rather than assuming “epoxy equals safe.”
Cost per gallon vs cost per finished piece
Polyester’s price advantage is real but easy to overstate. Per gallon, polyester is clearly cheaper - a Castin’Craft 32 oz kit runs about $25-$45, and bulk polyester from Fiberglass Warehouse lands around $80 a gallon, less per ounce than premium epoxy. Deep-pour epoxy kits like Fathom run $120-$450, and table-top epoxy $40-$160. But the right comparison is cost per finished piece, not cost per gallon. If epoxy’s low shrinkage, clarity, and strength mean fewer failed castings, fewer cracks, and a result you do not have to redo, the premium often pays for itself - especially on a single high-value river table where one cracked pour wipes out the savings. Polyester wins on cost when you are pouring volume, can tolerate some shrinkage, and the piece is not precious.
Mixing epoxy and polyester
A common question: can you pour one resin over the other? The bonding is one-directional. Epoxy bonds well to cured, sanded, and cleaned polyester - which is exactly why epoxy is the standard choice for repairing and laminating over old polyester fiberglass. The reverse does not work reliably: polyester is a weak adhesive and bonds poorly to cured epoxy, and polyester gelcoat over epoxy can fail to cure or peel. The rule is simple - layer epoxy over polyester, never polyester over epoxy. For a single project, the cleanest approach is to stay within one resin family from start to finish.
Project-by-project pick
- River tables and deep castings: deep-pour epoxy. Low shrinkage and 2-3 inch single pours win decisively.
- Bar tops, counters, and art panels: table-top epoxy flood coat for the gloss and the easy 1:1 mix.
- Coasters and food-contact items: a fully cured, food-contact-safer epoxy - never polyester.
- Jewelry and small clear castings: epoxy for clarity and low yellowing.
- Fiberglass lamination and boat repair: polyester for cost and speed (epoxy if you need maximum bond on a repair).
- Fast budget embedments where the piece is decorative: polyester for the 10-20 minute gel and lower cost.
Verdict
For nearly every modern resin-art and table project, epoxy is the right default and polyester the cost-driven exception. Epoxy’s low shrinkage, strong bonding, low odor, and food-contact-safer formulas make it the safer, clearer, more durable choice for river tables, coasters, jewelry, and art panels. Polyester earns its place only where its price-per-gallon and fast cure outweigh shrinkage and styrene fumes - chiefly fiberglass lamination and quick, decorative, budget casting.
For the full lineup of pressure pots, vacuum chambers, and deep-pour resins that go with these projects, see our best resin equipment buyer’s guide, and browse every head-to-head in the comparisons hub. The comparison table and per-product specs above give you the exact numbers to confirm the pick for your specific build.