Leveling a finish/finishing the finish

Question

I'm using Emmet's Good Stuff (poly gel varnish that you wipe on) as a finish on a walnut slab. I've read a lot about finishing online, but am a little confused about "leveling the finish." As I understand it poly varnish requires some minor abrasion between coats for good adhesion. The guy at my local woodworking store told me to use 320 to accomplish this as well as to knock out any streaks left by my application rag, dust nibs, etc in between coats. I take it that this is not the "leveling step" described in sources like Flexner's "Understanding Wood Finishing" and is really more of a very light hit with 320 so as not to "bust through" the finish? How many coats of finish should you put on before "leveling"? As I understand it "leveling" means sanding (possibly wet sanding with ~320 grit) until the entire surface is at an even sheen, which presumably requires some amount of build up or else you're going through to the wood.

Flexner's book (page 156 -- "Brushing Varnish") says: "brush on the first coat....sand the surface lightly with 280-grit or finer...apply the next coat...sand the surface lightly with 320-grit or finer sterated sandpaper...If you're trying to achieve a perfect or near-perfect flat surface, sand out the brush marks after the next-to-last coat...use 320-or 400-grit wet/dry sandpaper lubricated with soap and water or mineral spirits. The last coat will then level better."

Does that mean that if you apply, for example, 5 coats of finish that you sand the first one with 280, the next one with 320, don't sand the next one at all, wet sand the 4th coat, and polish the final 5th coat? If you apply 7 coats would that imply two more coats with no sanding/abrasion at all?

On a related note his book also warns about ghosting or going through one layer to another. If you're leveling a surface, which has brush marks in it are you not -- by definition -- taking off more material in some places than others, which means that you must be intentionally ghosting in certain spots?

After leveling I take it the next step is polishing/rubbing, which can be done with things like abralon discs and the like. Is that typically done on the leveled surface or on yet another coat over the leveled surface?

Answer 1

As I understand it poly varnish requires some minor abrasion between coats for good adhesion.

Nope. You'll read this online a lot (and in some books, and what's worse even in the instructions for some products) but it is completely untrue. The only reason to sand between coats is if you need to 'de-nib', or remove minor surface blemishes.

The important word above is IF. Where poly is spray-applied to a high standard for example in a dedicated spraying booth with filtered air — such as in many commercial operations — there would never be any sanding between the coats of poly and they don't have problems with poor adhesion.

The guy at my local woodworking store told me to use 320 to accomplish this as well as to knock out any streaks left by my application rag, dust nibs, etc in between coats.

320 may be a tad high but it is a suitable grit for de-nibbing.

There is some individual opinion and personal preference for the grit(s) to use for sanding between coats. Many woodworkers progress from 240 through to perhaps 400 as they sand the first and then progressive coats. There is likely some 'voodoo' or personal mythology here, given the nature of varnish and the fine scratches that even the coarsest of these grist will leave; for certain applications there is probably no discernible difference to de-nibbing with 240 or 400. Except that the 400 will clog faster and work more slowly.

As I understand it "leveling" means sanding (possibly wet sanding with ~320 grit) until the entire surface is at an even sheen, which presumably requires some amount of build up or else you're going through to the wood.

Dead on.

Not all guides to wood finishing will say this but flattening should always or nearly always take place just once, and as the final step after all the finish is down.

We can take a lesson from the way that finish is rubbed down on high-end cars here. There they only sand at the end, without exception unless something goes wrong. In woodworking one of the most common similar-looking finishes is on pianos, and there too they wet-sand only at the end (followed by buffing). The standards for flat and smooth in both cases here are generally far higher than required by most woodworkers, so these perfectly illustrate that sanding between coats is unnecessary to achieving a great result.

On a related note his book also warns about ghosting or going through one layer to another. If you're leveling a surface, which has brush marks in it are you not -- by definition -- taking off more material in some places than others, which means that you must be intentionally ghosting in certain spots?

This is more of a problem with certain finishes and not with others where the multiple coats are more cohesive. And yes you're right, it is practically inevitable that it will occur to some degree or another.

After leveling I take it the next step is polishing/rubbing

Correct. And yes this is usually on the levelled surface.

There is rarely any point to buffing or polishing an as-applied varnish layer because it is inherently glossy enough. The polishing is to regain the gloss lost by the sanding done during flattening.

The converse — rubbing down a final coat of varnish to reduce sheen — is frequently done however.