Question
I have been successfully using conversion varnish on furniture for several years and just recently began using pre-cat Magnalac lacquer. I assumed that pre-cat lacquer was like NC lacquer in that you could rub out and topcoat as many times as you needed to get your desired finish. Hence, I just finished an 8' white oak dining table in grey aniline dye/black wiping oil stain and about 7 coats of pre-cat tinted Magnalac lacquer. I used so many coats in an effort to protect the dark stain and to correct imperfections and an accident in the previous coats, although I sanded to 400g in between each coat. The surface looks great now and has been sitting for about a week, but several articles at WOODWEB suggest the finish might start to crack or peel in no time due to too many coats. Is this a possibility and why?
Forum Responses
(Finishing Forum)
From contributor D:
You used Magnalac, which is a coating that is not forgiving when it comes to dry mil thickness. If you have exceeded the 4-5 dry mils in coating thickness, then your coating may shatter. It develops fractures within the film. The durability of the coating is also affected when you exceed that maximum limit on how thick the coating should be. The 4 - 5 mils is a total coating thickness, not just the Magnalac part of the coating. Maganalac is a weak-performing pre-catalyzed lacquer. It is more durable than regular lacquer, but not that much more. The one and only great thing about Magnalac is how beautifully it flows out.
In your case, if you can start grinding that coating down, then you will be better off in the long run. If not, hope for the best. And if the coating does fail or if your customer complains about its lack of performance, then prepare to strip it off and refinish. This time around, either use plain old lacquer or a coating that enjoys a much better reputation (MLC coatings are fine except for Magnalac, which has a track record with way too many shops for failure).
In your case and in the case of many other Magnalac users, the failure of this coating is only the result of operator error. Not enough respect is given to the guidelines spelled out in tech sheets. There's nothing cut in stone that your coating will be doomed. Just be ready.
Did you thin your Magnalac? Magnalac is anywhere from 17% to 22% solids by volume. How thick were your applied coatings (wet mil thickness)? What about sealer coatings? Your scuff sanding… did you grind away a good portion of coating or did you only create a tooth?
Your coating may already be too thick. No, you cannot add conversion varnish on top of a coating which may already be too thick. In general, adding conversion varnish to an existing topcoat from a different system (precat lacquer, in your case) is inviting risks. The first risk right off the bat, assuming that you had some mil thickness to play with, is one where your CV topcoat causes the Magnalac undercoat to wrinkle.
Grinding back applied coatings of precat may invite wrinkling. If you sand through a layer and do not use a spot application of vinyl sealer, then your next applied topcoat can work its way under your sand through and cause what's there to wrinkle.
If you thinned your coatings enough, then you may not even be at the 4-5 dry mils of cured coating. No problem and no worry.
