Add resin to hardener, Add hardener to resin, or doesn't matter?

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Well-Known Member
Aug 30, 2006
St. Louis, MO, USA
I'm guessing that order doesn't matter since I don't recall seeing anything about it... but I figured I'd ask just to be safe.

In chemistry class, I remember being told to always "Add acid to water" because adding water to acid can cause excessing heating and, in the worst case, splatter acid all over the place.

Why do I ask? Well, I was daydreaming about building a "hot box" similar to Bernard Siu's 3rd version (scroll to the bottom of and thinking of ways to "easily" switch between hardeners. Then the thought came: If I'm using a system like MGS 285 where the hardeners can be mixed and I can safely add resin to the hardener, it's just a matter of adding a 3rd bottle and pump to his design. Want medium speed hardener? Then pump out a little of the fast hardener, add a little of the slow hardener, mix, weigh, pump and weigh out the appropriate amount of resin, and mix again.

There has to be a reason this is a bad idea or "somebody would have thought of that already.":whistle:


Well-Known Member
Mar 2, 2003
Western Washington
Personally I don't think it matters - usually it just seems to be an issue of convenience and/or ease of measurement. For instance, some epoxies have a very thick part A with a very fluid part B, and/or use a lot less part B. As such, it is much easier to dig up and measure out the first part and then simply pour in the smaller amount of hardener (Hysol EA9430 comes to mind). When the two have very similar viscosities or you use a 1:1 mixture, I wouldn't think it makes any difference.


Super Moderator
Staff member
Log Member
Oct 18, 2003
Saline Michigan

You hit the only reason I know about. Pouring water into acid can spash acid on you!

In epoxies, most of the things that can cause allergies are in the hardener, so you might take the same approach, but we are close to dancing with trivia here.

The big things are that the proportions are correct and that it is well mixed. When the two components have greatly different viscosity and density, this means a thorough stir of the entire container. In my shop, we use a full turn of the sweep second hand as our reference. Even if the color looks uniform, keep stirring and wiping the sides and bottom until one minute is up.

Some of the structural adhesives are terrible - One part is opaque and stiff like ice cream or peanut butter, and the other is like syrup. And you are supposed to get it throroughly mixed! Like you can tell by looking.

I have converted over to Proset here. Performance (strength, toughness, adhesion) is great. The stuff works like vaseline, is transparent with one component blue, the other yellow, proportion is 2:1 so weighing is easy, and when you have mixed it to a nice uniform green, it is ready to butter the joint.


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