Guitar Making Tip Number 973 is about the myth that you need to sand in between every coat of finish. While some finishes do benefit from sanding in between coats, in other cases it’s completely unnecessary. Here are the details.
Sanding Between Coats When Finishing
There are some finishes that are so solvent resistant that they don’t even bond to each other well in successive coats. Polyurethane is a perfect example.
In other cases, there are finishes that melt the existing layer and create one layer when dry. This means no matter how many coats are applied, they all melt into one.
Most finishes are somewhere in between, and depending on how the application process goes, you will not need to sand as often as you have been told. There is another reason that people sand in between coats, and this has more merit than the other reasons…
Wood Finishing and Mechanical Bonds
Most bonds between coats of finish are chemical bonds. That is, one layer chemically adheres to the next, keeping them together. A mechanical bond is different, which you can think of as interlocked fingers keeping your hands from pulling apart.
When you sand the surface of a finish, you create very small scratches. When you magnify those scratches, the look like little canyons with jagged edges and deep gouges. A liquid finish flows into those areas, hardens and holds.
Finish that gets into those areas and hardens is now holding in a second way. Not only is there a chemical attachment between the layers, but now there is also a physical or mechanical attachment.
Sand Only When You Need To
There are a couple reasons to sand in between coats. The first is when the finish calls for it right in the directions. On some solvent resistant finishes like Polyurethane, it’s a good idea to sand in between coats for the mechanical bond. On lacquer, that melts into one layer, it’s not as necessary.
Of all the reasons to sand, probably the biggest is when you apply the finish too thick. A thick finish causes runs, and those need to be sanded out. It also causes differences in thickness on the surface, which again requires sanding.
If you apply thin coats, which is The Secret to Wood Finishing, you will reduce the amount of times that you have to sand. This reduces the overall finishing time, and more than offsets the time that thin coats will add to the process.
Guitar Making Tip No. 973 Wrap-Up
Sanding in between coats of finish is a necessary process in come cases, and not in other cases. I recommend that you follow the directions as given by the maker of the finish, and you should not have any problems.
When in doubt, lightly scuff the surfaces between coats with steel wool, and you can get the best of both worlds. Apply thin coats throughout the process, and the final look will be flat and without large defects to remove with sandpaper.
If you have any questions about Guitar Making Tip No. 973 please leave a comment and I will be glad to answer them. Happy building.
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Tip No. 973 is from my book, 1,001 Acoustic Guitar Making Tips for Beginners, which has a thousand more great tips to get your first few builds on the right track. There is no secret to guitar making, just a lot of small things that you need to get right. These tips will help you.