Thinning Epoxy Primer

OK. I've made my primer decision.. It'll be PreKote and Sherwin Williams 2 part mil-spec epoxy... Minimize toxicity (PreKote), and get good corrosion protection (mil-spec epoxy).

My question to anyone who's used this epoxy primer (or another brand of similar stuff): In the info sheet, they say to mix the two parts 1:1. Then they say "reduce to spraying viscosity using slow or fast thinner (part no...), or a mix of the two to meet shop conditions" And then they say "reduce aprox 25% by volume to 17 seconds using a #2 Zahn / 45-65 ISO 2431"

Does it really have to be thinned, or is it good to spray as is? And what the heck is '#2 Zahn /45-65 ISO 2431', a viscosity measuring device? And is straining necessary (they say it is 'recommended')?

Thinning Primer

I don't use this particular brand of primer, but I can give you a general overview.

Yes, you will have to thin a 2 part epoxy to spray it. It will be much too thick to spray otherwise. Epoxy primers cure rather than dry and the 1:1 mixture gives the primer the correct proportions to cure properly. The reducer thins the primer to the proper consistancy to apply, whether by brush, spraying or whatever. Spraying generally requires a lower viscosity so that the spray gun can atomize the paint. When the reduced primer is applied, the reducer evaporates and the primer then cures due the the catalyst. The choice of a fast or slow reducer is generally based on the temperature when you spray. Fast reducers evaporate quicker than the slow reducers. In colder temps you use the fast reducer and in hotter temps the slow reducer. The goal is to get the mixture right so that it can flow out before the reducer evaporates, but does not have time to run. If it evaporates too fast you get an orange peel effect, if too slow you get runs.

A #2 Zahn cup is a viscosity measuring device. It is simply a small cup with a hole in the bottom. The cup is a specific volume and the hole is a specific size. The cup is dipped in the paint to fill it and lifted out. When the cup is clear of the paint, a stop watch is started and the paint drains out the hole in the cup bottom. As soon as the steam of paint starts to break up, the watch is stopped. This time is what is measured. The thinner the paint, the faster it streams out. My HVLP system came with a Zahn cup.

Straining is probably not absolutely necessary but it will remove any clumps of material that could cause the spray gun clog or to spit or drip, leaving a better finish.

Hope this helps.