Laminated a part yesterday and most of it's rock hard, but there are soft sticky patches here and there. It was a cold morning in the workshop. What have I done?
Soft, tacky patches in an otherwise hard part almost always point to mixing rather than a bad product. Epoxy cures by a fixed chemical reaction, so most laminating epoxy resins are mixed at a set ratio by weight or volume, and any resin or hardener that is not thoroughly blended stays sticky where the two never properly met. Scraping the sides and bottom of the pot, then mixing again in a second clean container, catches the material that usually gets missed. The cold morning will not have helped, because low temperatures slow the cure right down and can leave it feeling undercured far longer than you expect. Warm spots often harden in time, but genuinely off-ratio or unmixed areas will not, so scrape those back and recoat. Measuring the ratio properly rather than eyeballing it prevents most of this.
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