by Rick Hill
In finishing, one of the most frustrating problems can be finding the source of fisheyes. Also known as craters, these appear when some sort of oil or silicone has contaminated the finish of the wood. This contamination appears as a round crater or fisheye in the topcoat and is caused by a different surface tension in the finish than on the spot of contamination. In other words, the silicone or oil has more slip than the finish, so the finish pulls or slides away from the spot of oil, leaving a round looking "fisheye."
Here is a compilation of three fisheye hunting expeditions I have been on recently. These cases should help you to set up a few precautions to prevent or cure the problem in your shop.
Hunt # 1 White Enamel Doors
This shop is coating white enamel doors using a primer, sealer and topcoat over MDF. The fisheyes were appearing in the topcoat, so it was assumed that there was a problem in the finish from the supplier. This was not the case. Because the primer and sealer coats have a lot of fiber/grain to stick to, they won't slide off the oil spot. Fisheyes don't appear until you apply the topcoats or have filled the grain completely. That's why they always seem to be in the topcoat.
The shop has an excellent reputation for cleanliness in their spray area, because of all the white enamel they do. They are also very aware of how silicone problems happen, so I was surprised that the problem was occurring here. If you have sprayed enamels like white (or even worse, black) you know they show every speck of dirt and lint in the shop. Shops that spray a lot of enamel doors are often considered the better educated finishers in the market.
I asked the usual questions about fisheye: Had they changed the oil filter in their compressor line? Had they cleaned out the hoses and guns after the filter change? Are they using any silicone sprays or caulks in their shop for tool blade lubrication, or putty? Silicone is the worst offender, because it stays airborne in minute particles. It floats around the shop landing on wood, tools, and equipment. ( In another unrelated instance, a company that applied silicone caulk in laminate cabinets on one end of a 40,000 sq. ft. warehouse experienced fisheyes on the other end in the spray room. They eliminated the caulk and the fisheyes disappeared.)
After eliminating the most likely causes of the problem, I visited the shop for a fisheye hunt. They first thing I did was to dust the raw board with talcum powder (talcum powder will stick to the fisheye). No fisheyes appeared after dusting the raw board. If they had, I would have known that the board coming into the shop was contaminated from the board supplier. It is not uncommon for board mills to use tool lubricants that transfer from the cutting tool to the wood. These lubricants can cause fisheyes. Remember that most oils are a culprit, so things containing oil could cause the problem.
The in-house finishing system on this job uses an air compressor. Air compressors tend to leak oil and the oil filters don't always catch all of the contaminant. To eliminate this variable I cleaned the gun and line with a fast solvent like MEK. This gets all the oil out of the line and I know that I may have eliminated one possible cause. To further verify that the lines are clean, I bring a separate small turbine unit with my own gun and spray a few step boards with that system as well as a few boards with the in-house system. If fisheyes show up on the boards sprayed with the in-house system, but not with my turbine, I know the in-house finishing line is a possible culprit.
For the next step, I used a fresh can of finish from the same supplier on one board. On another board I used a can of finish from their shop (also the same supplier) and on a third board I sprayed the same type of finish from a different supplier. On the fourth board I sprayed the fresh finish I had brought, through my gun and turbine. All showed fisheyes. After spraying with both systems I put the step boards on the rack to dry.
A step board is a panel of wood that is taped off every few inches. Each successive step you do on the board is taped off to keep the next step from re-coating the previous one. Step boards are a physical sample used to verify each separate test you perform. (You may have seen step boards used by people doing stain matching to show each successive color layer.)
After drying, all of the boards showed fisheyes. Since all of the boards were dusted and sprayed out of a clean line and gun and fisheyes showed up in three different finishes, we knew the finish was not at fault. After eliminating the finish and the compressor as the cause of the contamination, what was left?
Well, we know that silicone stays airborne after spraying for a long time, so maybe it was floating into the room in some manner and landing on the parts. To verify this I walked outside of the plant and sprayed one board through my turbine and, after connecting a lot of hoses, I sprayed another board outdoors through their lines. Lo and behold no fisheyes!
Now we know where to look for contamination. We noticed that only the top few doors on the drying rack had been contaminated. As we looked up we saw the heating duct for the roof furnace was blowing directly onto those parts. By re-spraying one of the previous boards that had been sprayed outside and letting it dry on the rack under the heating duct we were able to determine that the fisheyes were floating in through that duct.
This shop had recently had a furnace problem. The furnace repair man had given the roof heating unit a tune up and used a silicone belt dressing on the belt. Every time the furnace would kick in, the air was forced right on to the drying racks and silicone was pushed onto primed MDF waiting to be topcoated. With a new dry belt and a change of ducting, the problem was solved.
Hunt # 2 Diesel Smoke
Hunt # 2 was simpler. As we were doing the step boards I noticed that the company delivery truck backed up to the loading dock and idled while we were spraying. Diesel smoke is filled with oil and as the truck idled it spewed diesel oil onto the wood. A reduction in idling and a vinyl curtain eliminated this problem.
Hunt # 3 Potato Chips and Lotion
Hunt#3 took a little longer, but we found out through step boards and talcum powder that the sporadic fisheye was appearing right after assembly of the wood parts. We found two causes, potato chip grease and lotion from assemblers hands. The company asked that the snacking on-line stop and everyone wore thin cotton gloves. The fisheyes stopped.
Hand lotion, wax, grease, oil, spray lubricants, diesel smoke, silicone, can all be culprits. One of the problems areas where I see a lot of wax is in the refinishing of furniture. Many strippers contain wax. This wax is used to hold the stripping solvents on the part long enough to take off the finish. Strippers often leave a waxy residue you can feel. If you don't clean off the part with solvent prior to coating, the wax will cause fisheyes.
A quick fix that cures fisheyes is to add a little "Fisheye Killer" to the finish. This additive is available from your finish supplier and will decrease the surface tension of the finish and let it stick over the contamination. This is only a Band-Aid to get you through production until the problem can be solved. Fisheye Killer slows the dry time of the finish, and as the contamination increases, you can't add enough to the coating to cure the problem. Keeping the finish open longer also allows more dirt and sawdust to land on the topcoat as it dries. Use it only to give you the time to begin the hunt, not as a cure.
If you come across this troublesome problem, talk to your finish supplier immediately.
Rick Hill is an independent representative and consultant for industrial wood finishes. He has been in the woodworking industry for 12 years, and has been known to actually hold, shoot and clean a spray gun.
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