Two parts, one reaction, and why the ratio is not a recipe

What off-ratio actually does

Too much of either leaves unreacted material in the finished piece. It does not cure a bit softer and then catch up; it cures with chemistry left over, permanently tacky or soft, and no amount of extra time fixes it. This is the single most common failure in the craft.

And it leaves uncured chemical in the piece

Which matters beyond appearance. Unreacted hardener in a finished object is sensitizing material somebody will handle, so an off-ratio piece is not merely ugly, it is a thing you should not sell.

By weight or by volume, and never both

Products specify one or the other and the two are not interchangeable because the densities differ. Mixing a volume-ratio product by weight gives an off-ratio pour that looks perfectly measured. Read which one the product uses and use that.

Mixing is part of the ratio

An accurately measured but poorly mixed batch has correct overall proportions and wrong local ones, which produces soft patches rather than a soft piece. Scraping the sides and the bottom of the cup, and moving to a second clean cup, is standard practice for exactly this reason.

Pot life is not working time

Pot life is measured in a mass in a cup, where the heat builds. The same mix spread thin in a mold stays workable longer. Treating the number on the bottle as a countdown for your pour rather than for the cup is how people get surprised.

Which is why the next page is about heat

The reaction releases heat, the heat speeds the reaction, and the cup is where that loop is tightest.

The small component is the one to measure carefully

In a system with an uneven ratio, a small absolute error in the lesser part is a large proportional one. That is where a scale earns its place over a molded graduation, and it is the half people eyeball because there is less of it.

References