One of my friends has asked me to tackle the subject of touchup on catalyzed finishes. John, for you, today is the day!
Catalyzed finishes are a group that has specific characteristics. The catalyst is there to improve performance. Improved performance is often at odds with repair-ability.
One of the directions that improved performance goes is toward increased chemical resistance. That resistance could simply be better resistance to something as bland as water. However, the really good post-cats become so chemically resistant within hours of application that a subsequent coat sprayed over the top will not rewet the previous coat. No chemical bond is possible; only a mechanical bond created by scuff sanding.
What I’m saying is leading us toward one of the refinisher’s laments. That being that the refinisher often has no idea how his lacquer will affect that which was applied before. It’s often a roll of the dice. Yet, here are some general rules that one should always follow.
If you know that the finish that you are repairing is a post-cat
This is actually the best finish to work with because that chemical resistance, inherent in most post-cats, means that you have a stable coating to work with. On the other hand, that chemical resistance pretty much means that spot repairs aren’t going to be easily hidden. A good scuff sanding and then recoating the whole panel is all that is necessary.
If you know that the finish that you are repairing is a pre-cat
This is trickier because you don’t know the level of chemical resistance of that pre-cat unless you know its exactname and characteristics. Neither do you know what will happen if and when your new coating application rewets the existing coating and combines with it. More on that in a moment.
Let’s use two pre-cat examples as points of illustration. M.L. Campbell’s MagnaMax and MagnaLac are pre-cats. However, they are as different as they can be in regards to their chemical resistance.
MagnaLac has a shelf life of around 18 months. MagnaMax has a shelf life of 120 days. The stronger the catalyst used, the shorter the shelf life will be. Look at post-cats. Their shelf life is measured in hours instead of days because their catalysts are very strong and they create a chemical reaction that is relatively short lived. In a post-cat we refer to that life as pot life instead of shelf life. But both terms refer to the envelope of time during which the coating is at its best from a performance standpoint.
Applying that rule of thumb to MagnaMax means that it should and it, in fact, does become more chemically resistant than MagnaLac. Lab testing has shown that MagnaMax will become very chemically resistant rather quickly and that scuff sanding is necessary between coats to create good inter-coat adhesion. It is going to be a tough one to spot repair because subsequent coats are not going to rewet and bond well.
MagnaLac, on the other hand, though it’s a good coating, is, by comparison, rather bland. Though scuff sanding between coats is always advisable, a subsequent coat will rewet the previous coat and create a chemical bond.
Another issue that impacts here is what I call the A + B = C reaction. That occurs when A is the coat already applied and it is different from B, the coat you are proposing to apply. If A is rewetted by B, then they will “intermingle” to some extent in their temporary liquid state. That marriage will dry as C. The consequences of that may not be immediately obvious and one can be assured that no chemist has tested what C’s properties are…good or bad so you are on your own here.
Also, let’s not forget the rule of thumb that one never wants to recoat with something harder than what is underneath. Consider here that you put a post-cat over a pre-cat. That pre-cat is going to have different characteristics than the post-cat. The pre-cat will be more flexible to a degree than the post-cat. Think of it this way. Think of the pre-cat as peanut butter and the post-cat as a plate of glass. If one were to spread a coat of peanut butter on a board and then lay a pane of glass on top of it, that pane of glass is now floating on the peanut butter and not bonded well toit or to the board. These layers are going to expand and contract at different rates and the glass, being harder, is going to be prone to stresses and impacts. This is where spider cracking, etc. come from.
More next week on questions to ask yourself before you decide to proceed with repairs or recoating.
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