Burned through finish while polishing

[kneelingyak]

I had my first major setback yesterday while I was trying to polish a finish. I've been working on a Crokinole board for awhile trying to get it finished by Thanksgiving. I was almost done, just polish the playing surface and a few other finishing touches, and I went straight through the finish to wood while trying to level the surface. At this point I'm just going to sand everything off (hopefully without going through the veneer on the plywood) and try again. But I would like to find out where I went wrong here.

A month ago I applied about 9 - 10 coats of a 50/50 wiping varnish. Some of the coats were fairly thick but everything still looked and felt pretty level before polish. I was following Bob Flexner and some other references that said I first need to level the finish to a completely flat sheen before going back up through the grits to the final polish.

I tried very carefully to sand with even pressure with the same number of passes across the whole surface. I started at 600 grit (with soapy water) as suggested by Flexner but was still left with these wavy shiny spots so I dropped to 400 and tried again. They were still there so I tried a bit of 320 and that is when I went all the way through. But the shiny spots are still there! Even right around the area where I sanded down to wood! Surely those areas should have gotten flattened?

Any ideas where I went wrong here (other than sanding too much)? Did I apply the finish incorrectly in the first place or was it just that unlevel that I could sand through some areas and still have glossy spots in others?

[AceHoleInOne]

Let me start out, I have not read Bob Flexner method's. However, I can tell you don't have enough finish down. Especially, if you thinned the wiping varnish down. Think about what polishing or rubbing a surface really is. Sanding the finish surface with higher grit imparts very small scratches and removes finish in the process. The smaller the scratch pattern, the glossier the surface becomes.

In-order to rub a finish, you have to have a sufficient build (mil thickness) of finish. Polishing or rubbing removes those layers of finish. If I'm going to rub a finish, I put down about 2 or 3 thick coats ( it depends, porous wood's will pull the finish down into the wood), I want to achieve a build, before I sand . Then I block sand to hit the high spots, this will level the finish and be careful not to sand through and keep off your edges. Then apply about 2 to 3 more regular coats and block sand between coats till you achieve a flat and level surface. Then polish your "flat and level" surface surface with your high grits till you achieve your desired sheen.

In conclusion, by applying 9 to 10 50/50 thin coats your actual film thickness is around 4.5 to 5 coats but what we don't know is how far the wood pulled the first few coats down. So it's very possible....you may only have 3 coats on the surface???????????

I hope this helps.

-Ace-

[CessnaPilotBarry]

The left photo looks like "witness lines" to me...

Witness lines are made visible when the finish is rubbed through several layers that have not properly bonded together. Crosslinking finishes, like varnishes and water based products, are most susceptible to this, as they usually have a very small window where the finish is uncured enough to allow the next layer to "burn in", or slightly melt the previous layer. Crosslinking finishes cure chemically, which is why they cannot be redissolved in their own solvent. This property also makes these finishes the most durable when it comes to chemical and food spills.

Evaporative finishes, like nitrocellulose lacquer and shellac, don't cure, they dry. Apply more solvent, and you can wipe the finish right off. This is true even years later... Due to this property, each coat of an evaporative finish melts the previous, becoming all one coat. This makes these finishes far easier to successfully rub out, and also repair. Since there's no free lunch, these evaporative finishes are less durable to chemical and food spills.

All that said... Most reactive finishes have a window where the next coat will best bond. Wait too long, and you end up with 9 distinct coats that can't be rubbed successfully. Recoat too soon, and you trap in solvents, causing other problems. For example, I have some varnishes that state a single 4-6 hour recoat period, an extremely fast drying product with a 1 hour window, and a "reopen period" a week later, and others that link quickly and should not be rubbed out at all. Some brands are far more consistent in quality than others, too...

Which product are you using, and what does the label mention around subsequent coats? Can you describe your application schedule?

I can try to offer suggestions. Unfortunately, with many reactive varnishes, often the only option is to sand back completely and reapply from step one.

[kneelingyak]

Thanks for the help guys.

I made the wiping varnish using 50% odorless mineral spirits and 50% Minwax fast-drying polyurethane (not the best I know but it is what I can get locally). The can says to recoat after 4-6 hours which I interpreted to mean that any time after that was ok. I guess that is incorrect. Flexner says to recoat after 4-6 hours or overnight but he also mentions making wiping varnish with a wide variety of products that probably have very different drying times.

I allowed each coat to dry for roughly 12 hours (one coat in the morning, one coat in the evening) with a light sanding of 400 grit before each coat. The shop was probably about 65 degrees during this process.

[CessnaPilotBarry]

I'd take it back to nearly bare wood, then proceed one of two ways.

- Brush full strength or lightly thinned varnish on the horizontal surfaces you plan to rub out, letting it self level, sanding with 320 or 400 between coats. You can wipe the rest of the item, but don't rub the wiped areas much. After a sufficient build is achieved, say 2-3 good coats, let dry fully and rub only the tops.

- Wipe on more varnish, and limit the rubbing to a light steel wool w/ wax. To do this, unfold 0000 steel wool and orient the fibers across the direction of the rub.

Contrary to many beliefs, advertising claims, and product names, polyurethane is actually softer than many other finishes. It's this flexible softness that makes it durable and damage resistant. The softness also makes it harder to rub out, as the finish "bends" rather than fracturing, as the abrasive moves across the finish. Give it a really good cure period, maybe a few weeks, before rubbing next time.

[kneelingyak]

Thanks for the advice Barry. I sanded it back to bare wood last night in order to get a fresh start. I'll probably go with your first option applying full-strength coats and go lighter on the rubbing. Hopefully it goes well this time and I can get it done in time for Christmas.