Re-Coat Timing and Pre-Cat Durability

A discussion of how quickly after applying one coat of pre-cat lacquer it is okay to apply a second coat. October 14, 2010

Question
We use MagnaMax for most projects. Lately, my finisher has changed his method a bit. First coat is with MagnaSand. Wait half hour or so and scuff. Then he's putting on a coat of Max, waiting only 10 minutes or so until it's flashed, and then hitting it again with the second coat of Max. No scuffing. Is there a durability issue here? Are we getting a drastically different result by not waiting longer and scuffing between the two topcoats? He's arguing that it's technically two coats, but skipping steps doesn't seem right to me.

Forum Responses
(Finishing Forum)
From contributor D:
Here's the way I have been doing things for years. I hit the product with a vinyl sealer, wait 20 minutes and scuff. I then topcoat the entire booth with max, which takes approximately 10 to 15 minutes. Then I hit the entire booth again (without scuffing in between). I have never had any kind of adhesion problems.

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From contributor U:
Surprised he isn't getting hazing or solvent blushing. The MagnaMax will flash off like a bowl of pudding and form a light skin, but in no shape or form have all the solvents escaped. Basically he is putting one coat on at about 9+ wet mils (if he is spraying 4-5 wet mils). This could lead to adhesion issues, and possibly create a brittle coat because the base coat is adhering to the substrate, whereas the "second coat" is just a buildup of film on top of that, which might melt in to a degree, but further from the substrate, the material can become less stable.

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From contributor L:
I don't see a problem with it. As long as you are putting the coats on before the crosslinking occurs, you shouldn't have to scuff. You can even do this with Krystal, but make sure you get the coats on before an hour has passed.

MagnaMax flashes quickly and the majority of the solvents off gas within 20 minutes. Once the majority of the gasses are done coming out, it is fine for another coat. I push MM frequently, doing coats 10-15 minutes after spraying. Never any problems. Never any blushing or wrinkling.

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From contributor A:
Doing the wet-on-wet coat increases your stack time. The longer you let your first coat dry before you lay down your second coat, the less time you have to wait to stack. The principle for evaporative is always that two light coats take less time to dry than one thick coat.

If you get your coating down quick enough, then you don't need to scuff sand between coats, and I think that this is what your finisher is looking to achieve, laying down coats within the window of time that allows for a proper chemical bond of the new coat into the previous coat.

Magnamax is formulated so as to avoid recoat window issues. That means that the solvent bite of the coating is developed to be weak and this is how they can avoid the wrinkling that you get when you create a recoat window issue.

This also means that it's always wise to scuff between coats rather than hazard a guess as to if you are spraying soon enough so that you are getting a nice chemical bite from your coating into the previous coat. The name of this game is intercoat adhesion.

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From contributor B:
Magnamax has a 3 hour recoat window, meaning if it is sprayed within 3 hours of its last coat, it does not need to be sanded in between. We have done this double coating for a while with good results.