Eliminating "Waviness" in a Polyester Full Fill Panel Finish

The trick is to make the panel dead flat before finishing. Here's advice on sanding methods. October 28, 2009

We are doing full fill high gloss polyester panels. The results have been good so far on the tests but a 4 x 8 sheet looks wavy in the light under the finish. We are sanding them to death after the build coats and then sanding 600 through 2500 grit before polishing.
I was told by our rep that this is the grain reflecting through the high gloss and is a characteristic of this type of finish. Is this true or is there something he doesn't know?

Forum Responses
(Finishing Forum)
From contributor A:
When you say "a 4 x 8 sheet", what is the sheet, plywood or MDF? What species of wood are you finishing? Sometimes a Cherry wood or Maple will leave a wavy look because the grain pattern of the wood telegraphs through the sheen of the finish. The glossier the finish, the more pronounced it will be. You might try block sanding the raw wood prior to building up your coating. If you are using plywood, the substrate itself is not true and you can sand the wood surface until it rains diamonds and it will never go away. If you are using an MDF core, you might try to block sand the raw wood as I mentioned, as that will help to eliminate the wavy look.

From contributor D:
I like to spray 2 separate coats of polyester on my high gloss finishes. I find that the first coat always tend to be a little wavy. I make sure I fill the grain completely with the first coat and orbital sand with 220 as flat as I can get it. Then I spray a second coat of polyester sealer which usually comes out very nice. I like to topcoat with 2k polyurethane because it's much easier to buff out but you could also just spray your final polyester topcoat also if that's what you need to do.

From contributor B:
Take a straight edge and see if the surface is flat. I suspect that the plywood was a little wavy. You would have to have some really crazy grain to cause this problem. I suspect you used a 6" random orbital to sand from 150-400. If there is a wave in the substrate the 6" disc will not remove it. You need to get enough film finish on the substrate to where you can take a dead flat piece of wood and use it to fair the finish. Once it's faired at 220, the rest will be easy.

Polishing a pile of poop is a fool's errand. Painting a pile of poop can be worth it. I learned this trick when fairing sail boat hulls that had to be sprayed with the highest gloss finish I've seen. It's a marine 2k called Awlgrip. We would use a fairing board to sand the fiberglass, mix up a batch of fairing compound (polyester or epoxy), mix red pigment, shoot the whole hull, followed by a coat of blue. It really lets you see the highs and lows when you start fairing.

From contributor C:
The plywood substrate must be as dead level as physically possible before spraying anything. Block sanding by hand is one way to do it, but you can save hours of time and much joint pain by using a straight-line air sand with hard felt pads (not rubber). National Detroit has been making the model #300 for around 60 years. I've had mine for 40 years and I wouldn't finish a piano without one. It is also excellent for leveling sealer coats and wet sanding. Random orbit sanders will not do the job. The soft pad follows the wavy surface of the wood, and it doesn't level the high spots.

After you've got the wood level, spray a couple coats of sealer, let it harden, then level with straight line air sander. Spray some topcoat, let it harden, look for waviness, and level sand again if you see it.

From contributor D:
As far as the substrate goes it should be sanded in a quality wide belt sander like a Heesemann. Anything else will not be good.