Epoxy Too Soft After Curing — Why It Happens and What to Do
Soft, rubbery, or flexible epoxy after the cure period is different from sticky epoxy — and has different causes and fixes. This guide covers the five reasons cured epoxy ends up too soft and the specific action to take for each situation.
Prevent soft epoxy by getting the mix ratio right before you pour.
Use the Mix Ratio Calculator →Soft vs. Sticky — What's the Difference?
Both are signs of an incomplete or incorrect cure, but they point to different causes:
| Symptom | Description | Primary cause |
|---|---|---|
| Sticky / tacky | Surface feels like adhesive; leaves marks | Too much resin (Part A) |
| Soft / rubbery | Flexible, dents under pressure, feels like rubber | Too much hardener (Part B), or wrong product |
| Gel-like | Neither solid nor liquid; jiggles | Severe ratio error, very low temperature, or wrong product |
Cause 1: Too Much Hardener (Part B)
What happened
Excess hardener leaves unreacted amine molecules dispersed through the polymer. These function as a plasticizer — keeping the network flexible instead of rigid. The more off-ratio the batch, the softer the result.
The fix
Soft epoxy from excess hardener will not harden further with time or heat. Remove the soft material with a scraper or grinder, clean the substrate, and re-pour with a correctly measured batch. Use the mix ratio calculator to confirm your Part A and Part B amounts before mixing.
Cause 2: Using Casting Resin for a Surface Coat
What happened
Casting resins are designed to cure deep — 1 inch or more — at a slow, controlled rate to manage exothermic heat. When used as a thin surface coat (under 1/4 inch), they may cure very soft, remain gel-like, or produce a permanently flexible finish because there isn't enough volume to sustain the exothermic reaction.
The fix
Use a tabletop or coating epoxy (specifically rated for thin surface applications) for countertops, tabletops, and floor coatings. Casting resin belongs in deep molds and river table channels, not surface coats. Remove the soft casting resin and re-pour with the correct product type.
Cause 3: Temperature Too Low
What happened
Cold temperatures slow the curing reaction, producing an under-crosslinked polymer that is soft, flexible, or gel-like. Unlike a ratio error, temperature-related soft cure may be recoverable.
The fix
- Move the project to a warm space (72–80°F / 22–27°C)
- Wait 48–72 hours at the correct temperature
- If the epoxy was correctly mixed, it may still firm up once warm
- If it doesn't firm up, remove and re-pour with fresh material at the right temperature from the start
Cause 4: Under-Mixing
What happened
Insufficient mixing leaves pockets of unmixed resin or hardener. Areas of excess hardener cure soft; areas of excess resin cure sticky. The result is an uneven surface with soft zones mixed with (possibly) harder zones.
The fix
Sand down the affected area fully. Re-pour with fresh epoxy mixed for at least 3–5 minutes, scraping the sides and bottom of the mixing container throughout. The "double cup" method — pouring the mixed epoxy into a second clean cup and mixing again — eliminates unmixed material at the sides of the original cup.
Cause 5: Using the Wrong Product Entirely
What happened
Some products sold as "resin" are flexible or semi-flexible by design — UV resin for flexible molds, polyurethane casting, or silicone — not the rigid epoxy needed for tables and floors. If the product is supposed to be flexible, it will stay flexible no matter what you do.
The fix
Read the product's Shore hardness or Barcol hardness rating. Rigid tabletop epoxies typically cure to Shore D 75–85 or Barcol 35+. If your product is rated for Shore A (soft/flexible) or isn't rated at all, it may simply be the wrong material for a hard-surface application.
Post-Cure: Can Heat Fix Soft Epoxy?
Post-curing in an oven or with gentle heat can improve hardness in correctly-mixed epoxy that is slightly under-cured from temperature. The process:
- Allow the epoxy to cure at room temperature for the full specified time first
- Place the piece in an oven at 140–160°F (60–70°C) for 2–4 hours
- Let it cool slowly to room temperature
Post-curing does NOT fix ratio errors. If the epoxy is soft because of excess hardener, heat will not produce the missing cross-links. It only helps correctly-mixed epoxy that needs more energy to complete its cure cycle.
Frequently Asked Questions
Why is my epoxy soft and rubbery after curing?
Most likely cause is excess hardener (Part B) relative to resin, which acts as a plasticizer. Other causes include using a flexible casting resin for surface work, temperature too low during cure, under-mixing, or using the wrong product type. Check the mix ratio first.
Can soft epoxy harden over time?
Only if the ratio was correct and the softness is from cold temperatures — warming the piece may let it complete curing. If the ratio was wrong, the epoxy will not harden further. Wrong-ratio epoxy must be removed and replaced.
What is the difference between casting resin and tabletop resin?
Casting resin is designed for deep pours (1+ inch) with a slow, controlled cure. Tabletop/coating epoxy is designed for thin surface coats (1/8–1/4 inch) and cures to a harder, more rigid finish. Using casting resin as a surface coat produces soft, gel-like results because it needs depth to cure properly.