Question(WOODWEB Member):
So if you're applying coats quickly, the only reason to scuff is to smooth the surface. You're going to scuff the seal coat because the surface will be rough. However, if the first topcoat doesn't need to be smoothed (denibbed) and you'll apply the second coat within hours of the first, there's no reason to scuff. Is this correct? If using pre cats, after what time period is scuffing required to promote adhesion? 24 hours? 48 hours? Same question for post cats?
I know one reply will be to check with the product supplier, but I honestly believe a lot of their responses are designed to cover their fannies.
Forum Responses
If you want to use your own judgment and go against those recommendations, you are taking responsibility for any adhesion problems that may result. That said, there are times where we have all pushed the limits to some degree at our own risk.
As a test, take the thinning solvent and put it on the dried coating (on a sample piece). If it damages it, then you can just recoat without scuffing. If not, you can't.
If the manufacturer says scuff, then the warranty requires scuffing. But then again, most warranties from manufacturers will replace the coatings and not the labor, so they are essentially worthless.
I'm pretty certain I read somewhere that scuff sanding between coats of pre-cat, and perhaps post-cat, lacquer wasn't an absolute must for adhesion as long as the coats were applied within a certain timeframe. Let's say the recoating will happen within a 24 hour period.
(Finishing Forum)
From contributor B:
The answer is simple. Scuff sanding is necessary when the coatings manufacturer says it is. If the guidelines on the TDS or back of the can are not clear enough for a given situation, the chemists/coatings engineers are usually happy to discuss this in greater detail, if you call the manufactures and ask to speak to tech support.
From contributor L:
If you shoot a post cat about 1 hour after the first coat, it should burn in. After the 2 hour mark you may have problems. With pre cats you probably have about 8 hours until burn in might not occur.
From contributor B:
Unless you're confident enough to bend the rules, it is best to follow the manufacturer instructions. It is less about the manufacturer's token warranty coverage than reducing potential for liability, or failure for that matter. If a failure were to result in a lawsuit, the opposing expert witness would certainly call into question what inter-coat sanding procedures, and other manufacturer guidelines, the finisher observed. This is getting into a situation that none of us should ever have to face, and you would probably be screwed regardless of what you did. But it serves as an example of what risks are being assumed when you start to write your own rule book.
From contributor S:
It has to do with the resin, but mostly the solvents and their ratio in the supplied container, the strength of the catalyst and finally the solvents the user may or may not add. Weather conditions if not controlled in the user facility can often be a crucial factor. Rule of thumb from more than 1 chemist - if the coating is dry enough, scuff it. Ask the supplier's qualified tech rep or chemist.
