Multiple Coats of Reactive Finishes
Two-part finishes don't "burn in" to previous layers of the same coating. Here are tips on careful scuff-sanding and re-application for building up the finish on a fine piece such as a musical instrument. February 12, 2007
Question
Does the Krystal Clear from ML Campbell melt into each layer like nitro, or does it lay on top of each layer like poly?
Forum Responses
(Finishing Forum)
From contributor T:
Krystal needs to be scuffed between coats so you get a mechanical bond. It doesn't "melt" into the earlier layer like evaporative finishes (shellac, NC lacquer). Krystal is a reactive finish (2-part finish) so needs this scuffing between coats.
From the original questioner:
I spray a medium coat onto the guitar every two hours and after each day, I scuff sand and spray more in the same time line (since it dries longer and has time to cure). Is this the best way to do it or is there a more efficient way? Also, after there is enough finish on, we level sand, and repeat that process until it's done.
From contributor T:
Gotcha. How many coats total are you putting on? You should know how much the dry mils is so you don't exceed the manufacturer's maximum, otherwise you could get problems. That said, you don't have to wait an entire day to do your next coats. When I use Krystal and am building up the finish (so it is grain-filled), I put one coat on, let it dry about an hour, scuff it and do another medium to heavy coat (4-5 wet mils). About every 2 hours after that, you can sand it and recoat. To get the fastest fill, I sand with an orbital sander and then a cork-lined block by hand so as to more aggressively knock back the finish, otherwise you'll just add coats and coats. You have to be careful with the orbital, as it could cut right through (especially after the first coat), so only use the machine sanding when you have sufficient build already (2-3 coats). This sanding takes off most of the finish you sprayed on ("knocks back"), but leaves it in the recessed areas of the grain. You'll get the hang of it and know your piece after a while. This way I can fill the grain and have a good build in about 3-5 coats with a perfectly flat surface, and know I haven't exceeded the maximum dry mils as well. And I can do it one day, two days max.
From the original questioner:
Is there a trick/tool to finding out how thick it is once it's dried? Other than, of course, experience (one thing I lack).
From contributor L:
The only way I know is destructive. You need to get a depth micrometer and scrape off your finish until you reach wood. Then you take your measurement. The maximum dry film for Krystal is 4-5 mils. I believe it has a 42% solid content and you can put 3 coats of 4-5 mil wet and still stay within the limit. This includes the sanding in between coats for those of you who are planning on adding up 42% three times and getting 126%.
