Post-cat lacquer vs. conversion varnish
A post-cat lacquer typically requires the addition of a separate catalyst by someone other than the manufacturer. Once this catalyst gets added, the lacquer starts a shelf life of anywhere from four to twelve months, depending on its acid value content reacting with the other components in the formula. Within this timeframe, the lacquer will gradually increase in viscosity and may not atomize properly through your spray equipment.
This differs from a pre-cat lacquer, because the manufacturer incorporates the catalyst at the point of manufacturing, so it's built into the formula. A built-in catalyst can lead to potential problems if the manufacturer and/or distributor do not rotate stock properly, insuring fresh material.
A conversion varnish also requires the use of a separate catalyst added out in the field just as a post-cat lacquer would, but the main difference is in the time you as an end-user have to apply the product. Most CV products come with a stipulation of "use it or lose it." This refers to its pot-life, which is on average eight to twelve hours. Post cat lacquers offer a shelf life in months.
In reality, not much. Post-cat lacquer and CV are very similar with the exception that post-cat lacquer has nitrocellulose in its formulation (which eventually will turn yellowish) and conversion varnish normally doesn't, with its primary resin system being an alkyd amino. Both use sulfonic acid as the catalyst to cause the cross-linking, which produces a very tough and chemically resistant film using either system.
In my experience, post-cat lacquer has nothing close to a several month pot life. Pre-cat lacquers fall into this time frame, but post cats' typical pot lives are much shorter. The pot life after catalyzation of a post-cat lacquer typically does not extend past 30 days. With conversion varnish, it's as short as eight hours and as long as 48, depending on the manufacturer. Also, the resins in a good water white C-V (Rel-Plaz, Global Resistovar, MacLac CV-4000 series, Sherwin-Williams Water White Conversion Varnish) do not yellow over time. A very important consideration over pickled work.
I'm a big believer in post-cat products both from the standpoint of shelf life and performance. I don't find adding the catalyst a big deal (I use hypodermic syringes for this purpose) and the fact that the post-cast products have virtually an unlimited shelf life until catalyzed makes them very attractive to me.
I find little difference in spraying conversion varnish from pre-cat except that the conversion varnish, because of its higher solids content, requires me to use a smaller N/N set in my gun. In short, post-cat lacquer or C-V are, in my opinion, superior to pre-cat.
Gemini Coatings makes a conversion varnish that has roughly a 21-day pot life after catalyzation. I have not used this conversion varnish, but they tell me that their customers who do use it think that it's a good conversion varnish and that the long, long pot life makes their lives easier. They also make one with a seven-day pot life. That also sounds mighty user-friendly to me.
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