Sanding Exterior Wood? Don’t Make These Costly Mistakes.

Sanding exterior wood the right way is key to getting a smooth, long-lasting finish—especially when using a high-performance product like Osmo. But many DIYers and even pros make simple mistakes that can lead to poor absorption, uneven colour, or premature wear.

In this post, we’ll cover the most common sanding mistakes people make before applying Osmo exterior wood finishes—and how to avoid them for the best results on decks, fences, siding, and more.

 

1. Using Grit That’s Too Fine

Mistake: Final sanding wood to high grits like 240+ because they think smoother is better.

Problem: Sanding too high a grit closes the pores of the wood, which prevents the finish from penetrating the wood surface properly.

Result: The finish can sit on top of the wood surface like a film instead of bonding deeply, leading to poor finish adhesion and peeling.

Best Practice: For most exterior projects, stop at 80–120 grit for good absorption without roughness. We suggest P80 for decks and P120 for furniture.

 

2. Skipping Sanding Altogether

Mistake: Thinking a power wash or light scrub is "good enough" and skipping sanding when finishing for the first time with Osmo

Problem: Dirt, old coatings, mill glaze (a shiny film from manufacturing), and weathering stay on the surface.

Result: Patchy colour, poor adhesion, and reduced durability.

Best Practice: Always prepare the surface properly and do not skip sanding before finishing

 

3. Uneven Sanding

Mistake: Sanding harder in some areas and barely touching others.

Problem: Creates blotchy absorption of stain or finish.

Result: Dark and light patches appear once the finish dries.

Best Practice: Sand with even, consistent pressure across the entire surface and ensure to sand in the direction of the wood grain

 

4. Not Cleaning After Sanding

Mistake: Forgetting to remove sanding dust before applying the finish.

Problem: Dust particles get trapped under the finish, ruining the look and adhesion.

Result: Cloudy, rough finish, and poor adhesion of the finish, potentially causing peeling

Best Practice: Always vacuum, wipe with a tack cloth, or blow off all dust thoroughly before finishing.

 

Quick Golden Rule:
"The goal isn’t to make the wood glassy-smooth — it’s to clean, even out, and open the wood just enough for the finish to properly bond."

EXPLORE OSMO EXTERIOR FINISHES

 

We teamed up with Mirka, leaders in finishing products, to put sanding grit to the test—does a higher grit improve hardwax-oil absorption on wood? To find out, we sanded surfaces to 120, 180, 320, and 400 grit before applying two coats of Osmo TopOil. Mirka’s lab team then analyzed oil penetration at each grit level, uncovering which provided the best absorption. Why does this matter? Better absorption leads to greater durability and stain resistance. Watch to see what they discover under the microscope!

WATCH ON YOUTUBE