Scuff Scratches Between Coats of Pre-Cat

      With catalyzed finishes, it works best to apply successive coats before the previous coat is fully cross-linked, so the coats melt together. tober 28, 2008

Question
I sprayed a final coat of ML Campbell Magnamax pre-cat on a set of doors approximately 3-4 weeks after my washcoat, stain, seal coat, etc. and I'm seeing all of my scuff marks underneath. Did I go too coarse (I used 320) considering the curing time allowed? Is there a way to correct this without stripping them down and starting over?

Forum Responses
(Finishing Forum)
From the original questioner:
The more I think about this, the more I'm believing that my final coat just wasn't heavy enough to fill all the scratches on the surface and that an additional coat may take care of things. Does this sound reasonable? I always need to thin the pre-cat slightly for it to spray properly from my gun so I don't think I'm in any danger of overbuilding the finish.



From contributor J:
It sounds like the seal coat has completely cured, not allowing intercoat melt in. With any finish that cross-links, there is a certain amount of time before the cross linking has taken place, in which every coat will melt into the previous coat just like an NC lacquer.

When the seal coat has cross linked, the top coat lays on top of the seal coat, so the sanding scratches will not melt in and end up "telegraphing" through the top coat. Thinning the finish may make this worse, but I doubt it is the main cause.

This is more common with post cat CV than pre cat, but Magnamax cross links like CV, it just does it slower. It's the three week time span at work here. Another coat may or may not help hide them, depends on the color/viewing angle. 320 grit should be fine under normal conditions, but sandpaper varies and if the finish "corns" up on the paper, this can make deeper scratches.



From contributor R:
I agree with Contributor J. I just sprayed 33 shelves (cherry wood) with Magnamax. My first coat was Magnasand, followed by two coats of Magnamax Satin. I used 3M 220 Tri-mite sandpaper and 3M's Maroon scotch brite on the Magnasand prior to the first clear of Magnamax. Plenty of scratches were evident before my first topcoat. After the first topcoat, the shelves were like glass and without any sights of those scratches. Note that this was done within three hours of each coat. Naturally, my advice is to seal, sand and apply the first top coat within eight hours day.


From the original questioner:
Well I gave them one additional coat (this time scuffing with a finer synthetic pad instead of 320 grit) and the scratches have disappeared. I am very relieved to say the least but I think from now on I will be sure to allow enough time to finish things complete in one day’s time.



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