What goes wrong

It stayed tacky

Updated October 2026

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Tacky everywhere means the ratio Tacky in patches means the mixing Tacky only at the surface It does not fix itself What to do with it Record the batch Do not sand it to see if it is better underneath References

A piece that never hardened properly is almost always a measuring or mixing failure, and where the tackiness sits tells you which one.

Tacky everywhere means the ratio

Uniform softness across the whole piece points at the proportions. Check whether the product specifies weight or volume and whether you used that one, because measuring a volume-ratio product by weight gives a confident wrong answer.

Tacky in patches means the mixing

Soft areas next to properly cured ones mean the proportions were right overall and wrong locally. The usual cause is unmixed material from the sides and bottom of the cup going into the mold. Scraping thoroughly and decanting into a second cup fixes it.

Tacky only at the surface

Often a cold or humid cure, and with some systems moisture at the surface during cure produces a greasy film. It can also be inhibition if something the resin touched interfered with it, which is the silicone failure arriving in a different material.

It does not fix itself

Extra time does not complete a reaction that has no partner left to react with. Waiting a week on an off-ratio piece achieves nothing, which is hard to accept and is the truth.

What to do with it

Usually scrap. A thin tacky layer can sometimes be removed and the piece recoated with a correctly measured batch, but a through-soft casting is finished. Do not sell it: uncured material at the surface is sensitizing material somebody else will handle.

Record the batch

Ratio method, the room temperature and which cup. Three columns, and they turn a recurring mystery into a pattern you can see.

Do not sand it to see if it is better underneath

Sanding uncured resin makes dust carrying reactive material and spreads the exposure rather than removing the problem. If the surface is tacky, the thing to do is work out why, not abrade it.

References

29 CFR 1910.1200, Hazard Communication. Paragraph (f) sets what a shipped container must carry: product identifier, signal word, hazard statements, pictograms, precautionary statements and the name, US address and US telephone number of the responsible party. Paragraph (g) requires a safety data sheet, in sixteen numbered sections. law.cornell.edu

16 CFR 1500.3, definitions under the Federal Hazardous Substances Act, which reaches products sold to consumers. A hazardous substance includes a “strong sensitizer”, defined as “a substance which will cause on normal living tissue through an allergic or photodynamic process a hypersensitivity which becomes evident on reapplication of the same substance.” law.cornell.edu

Written from federal regulation and published safety guidance, not from a laboratory. Epoxy sensitization is permanent and cumulative, and nothing here is a substitute for the safety data sheet for the specific product you are using or for a conversation with your state about what you may sell. Read the SDS for your resin, not a summary of somebody else’s.