Questions: Post Buffing Finish Scratches

by Brian Boedigheimer

Originally published in American Lutherie #86, 2006

 

See also,
Questions: Power Buffer For Finish by Charles Fox

 

Brian Boedigheimer of River Falls, WI responds to Mario’s question in AL#79 regarding what to do about tiny scratches in finish noticed after buffing:

Charles Fox responded earlier to discuss lighting and I certainly agree lighting is key in so much of the work we do. Another thing to consider is the grading system of the abrasives one chooses to level the finish. There are three systems: CAMI, which is the U.S. standard grading system; FEPA, the European standard; and Micron grading. FEPA P800 is equivalent to CAMI 400. The P indicates the FEPA grading system, which is important because the grit particles are more uniform than in the CAMI system. Therefore there’s less chance of a stray large particle causing a scratch that we won’t see until buffing. So P800 gives you the cutting action of a 400 grit without the stray (larger) particles of the CAMI rating. Micron has the tightest tolerances for particle size, and if you have access to Micron-graded paper, that’s even better than FEPA. The equivalent Micron grade would be around 25 or 20. I recommend the FEPA grading system for abrading coatings until you’re into compounds; then you’re dealing with Micron.

When sanding to level finish, the coarsest grit I would use is P400 grit, but typically I start with P600 or higher. I used to go through the process level-sanding with 320, 400, 600, and so on. The problem is, if you miss a 320-grit scratch working through the higher grits, you won’t know it until you buff out the guitar, and only in the right light will you see it.

Now when I level-sand my finish before the final coats, I use P800 and spend just a little extra time with that one grit. It really doesn’t take me much longer, and I know those tiny scratches won’t suddenly appear. Then I spray the final two or three coats. Once that’s cured, I only have one grit of wet sanding to do before buffing. At that point I choose P1200.

Don’t think that because each layer of nitrocellulose lacquer melts into the previous one, you won’t have stray 320 scratches show up. Even with that specific bond, a stray particle scratch from 320-grit CAMI-graded paper will show up when buffed out. I’ve experienced it. ◆