On Tuesday, October 16, 2018 at 8:39:42 PM UTC-5, -MIKE- wrote:

Oak has very wide grain and deep pores.
The poly will follow these hills and valleys like snow falling on bumpy
ground.
The only way to get a smooth, perfectly flat surface finish is to level
sand and build coats.

This is done all the time with lacquer, but I don't know if it works
with poly. The problem with poly is that each successive coat sits on
top of the one underneath. So if you sand through a coat, you get
rings/lines
like a topographical map showing where the top coat is worn through.

With lacquer, each sort of "melts in" to the one underneath, creating,
in essence, one thick coat instead of several individual coats built up,
so it's easier to level sand to fill pores and grain.

I'm not a finish expert and I'm not very good with poly, so there may be
a proper technique for filling pores and grain and level sanding with
poly as well.

I also may not have all of the exact details correct, but I'm pretty
sure I got the gist of it right.

Actually... you got all of it right! Build coats with lacquer? Check. Witness lines from sanding through coats of poly? Check. More effort than it is worth to build enough product to smooth out porous nut woods? Check.

Today's poly has a place in everyone's arsenal. My favorite finish to spray is a cross-linked polyurethane conversion lacquer, and you can build that.. But polys from the store... in most cases, no. There are commericial product available like ML Campbell that you can build, but they are not for the inexperienced. Even with the folks that know what they are doing (ahem....) you just can't reasonably get enough product down to fill pores and leave a smooth surface.

In the old days, you made an applicator and applied different fillers before using a finish. Now, we use Behlen's Pore o Pac (sp?) and follow the instructions. It is actually a superfine crystalline structure (most likely a super fine pumice or something similar)that is a used before applying finish. Applied as per instruction, it works great under just about any finish..

BTW, building poly is a fool's errand. Read the instructions on the product about application thickness, times to recoat, etc. Almost everyone I know including my "pro" coworkers apply poly incorrectly. If you miss your window to recoat, as you noted, you will get witness lines that show the coats. Poly will resolvate (in its window) just enough to grip the underlying coat. Second coat is normally in 8 hours. So if you put a coat on in the afternoon, then put a second coat on the next day you have already compromised your finish. And since you cannot "build" coats of poly, it is recommended to stop at two coats, with a 4-5 thousands thick coat each time. More coats or thicker coats will not allow the poly to outgas so that it can proper cure.

Sand, put on the pore o pac, then finish as normal.

Anyway... good post! Nailed it!

Robert