Brian,
This could turn out to be one of my short answers that takes about four pages…
From my experience, I built and finished woodwork pieces in custom vans back in the early 80’s. My primary job was the custom paint work on the outside, the woodwork was simply a bonus.
What you use depends entirely on what finish you want to achieve and where you are starting from.
Your assessment of the difference between a a dual action polisher ( random orbit) vs. a high speed or variable speed buffer is absolutely correct. I’ve used both, my preference and the only recommendation I’ll make for woodworking is to use the random orbit, or DA polisher.
My issues with the rotary buffer involves it’s tendency to burn through your finish and actually scorch your corners before launching your work into the next zip code… Or the heat generated with a high speed burnish that is expected to dissipate on an automotive application will actually blister your finish, causing a rework setback that is very time consuming.
The random orbit buffer is not recommended on automotive finishes for the medium cut product because the slower gentler action does not “break down” the polishing grit in the compound. This is important, more in a moment.
Compounds typically have either a water soluble base or a petroleum base. Be careful that your choice does not have a reaction with your finish.
Years ago in the automotive world, Lacquer paint was the standard finish. to achieve a high gloss and high definition of image (DOI), or depth of reflection, several coats of lacquer paint were applied, the solvent evaporated out, and the paint dried. The spray guns applied the paint in very fine droplets but left a paint surface / finish that was similar to the peel of an orange. Additionally, because the volatile organic compounds in the solvent evaporates out, the paint surface often had microscopic pores that reduced the gloss of the finish ( Imagine your living room carpet and how poorly it reflects images.) New urethane finishes and clear coats in the automotive world greatly reduce these issues,but the principles are similiar.
To achieve a high DOI, the finish had to be leveled, often with progressively finer grits of wet and dry sandpaper. As late as 1980, the finest grit paper you needed or used was a 600 grit paper. This left the surface relatively flat, with tiny scratches. Here’s where your compound comes in…
An automotive polishing compound is designed to break down into smaller or finer particles during use. the effect is like having a piece of sandpaper that starts out as a 400 grit that changes itself into a 2000 grit paper while you’re using it. There’s an old rule of thumb that I still teach apprentices that the “compound cuts while it’s wet, but begins polishing as it dries”. This is true whether you are using a wool or fiber wheel or a foam pad.
The next step take up where the prior step left off, essentially starting to cut at a 2000 grit stage and finishing out to around a 3000 or finer finish. This leaves an ultra smooth but still ‘warm’ finish. The highest definition of image is achieved with a swirl remover or final glaze that fills in the tiniest of scratches for a ‘wet look’.
If you are using a polyurethane finish, I recommend a light wet sand with progressive steps of 600, 1000, 1500 to 2000 grit automotive paper, and your second option of medium cut, fine cut, and either swirl remover or their finish hand glaze, or yellow wax.
Q
-- I don't make sawdust...I produce vast quantities of "Micro Mulch."
