Boat repair as it relates to contact lens polishing

Our Bladder Polishing System has seen wide acceptance. With each lab purchasing single or two spindle bladder polishers, we are asked for instructions and guidance on how the system works and how to address particular lens designs.

The beauty of the bladder system is that it works on all designs.I recently had an experience that, I think, relates to the differences between conventional polishing and the bladder system. I use the term "bladder system" because the bladder polishing tool is one element of the machine that does the job. It illustrates the different approaches needed to correct poor base surface quality and precision surfaces that just need to be buffed.As some of you know, I was raised in a boat yard which continues to operate.

Our yard manager called about a boat that he was repairing that he was having problems fairing (correcting) a surface gouge. I looked it over and saw that the final coat of dark green paint was not smooth and had brush marks and other surface problems. He was working on the boat outside. The cool wet weather was slowing the curing of the paint allowing it to sag.I started sanding it out with 320 (45 micron) grit wet sand paper and water to a uniform finish and proceeded to 600 (30 micron) grit.

This was done with a firm closed cell foam block as a sanding pad. I assumed that he had first lapped the surface with a wood block and sanded the white primer before applying the finish coat. That will be the kicker to the story.After sanding with the 600 grit, I washed it off and dried it. This showed up scratches from the 320 paper, so I went over it again with 600 grit paper and then washed and dried it. It took one more sanding with the 600 grit to eliminate the 320 scratches. Then I used 1200 (15 micron) grit in the same manner-washing and drying after each pass. I spent about an hour on this area which was about 200 X 400 mm.

There was still a noticeable difference in the original paint and the new coat, even though they were from the same can.So I was ready to buff the area with a rotary tool with polishing compound. The buffing brought the surface up to a nice shine (better that the original paint), and also blended in the color. I figured that I was about finished and had the manager look it over. We found a small area that wasn't fair, so I went ahead and sanded it with a wood block backer. That was my undoing. To fair the surface, a rigid backer is needed to cut down the high spots without removing material from the surrounding area.

That is what should have been done with the white primer. Well, you guessed it, as I was sanding the bump, a little white spot appeared which was a drip in the primer that the foam pad didn't fair in. The white primer was sanded with a foam block which did not fair the surface.The analogy to contact lenses is this: If you start with a poorly lathe-turned surface, you must lap it with a rigid tool (pitch) for at least a minute to correct it. The correction will make it spherical, but it may be off radius.

Of course, that only works on spherical lenses. You can buff a surface in 5 seconds if it is turned on a CNC air-bearing lathe. This works for any design.  The key here is that the surface is precise and doesn't need correcting. The bladder system buffs the surface to the required quality.Some Labs use the bladder polisher for non spherical base curves only while others use it for all base curves. It's amazing to me that some labs feed one single spindle bladder polisher with three lathes at 300-400 lenses per day.

The technician uses the same bladder changing it only for very steep base curves such as keratoconus. It used to require a few six spindle polishers to handle the same volume for spherical base curves only.Specialty lenses are becoming more common, but don't forget that the bladder system works well for spherical lenses, too.