Raka has 3 different kinds of hardeners, which cure at different speeds. Like System 3, West, MAS and others, they can be blended together to tailor the material to the ambient temperature. Unlike the others however, Raka hardeners use different ratios, and therein lies a problem.

With the others, you can interchange the various hardeners at the same ratios. Raka has a slow hardener (606, nonblushing, and highly recommended) and a fast one (610, which can blush especially at low temp and high humidity) which both mix at two parts resin to one part hardener by volume. But their medium-fast hardener (631, also subject to blushing) mixes at 5 parts resin to one part hardener. So 610 and 606 are interchangeable, but 631 is not. Very confusing. I suppose you could work out the math and get a stoichiometrically correct mix, but odds are against it. Here is the quote from the Raka website:

"The hardener 631, is a medium fast curing agent that can be mixed at a ratio of five parts resin resin to one part hardener. 631 gives the most strength and water resistance. Our best selling hardeners are the 610 which is very fast curing and the 606 which is slow curing. They are mixed at a ratio of two parts resin to one part hardener. The two to one ratio epoxies are probably the most popular because they give a good compromise in ease of use, are low blushing and have good overall physical properties."

What a PITA!