Air Bubbles in Finish

Air Bubbles after finishing Stair Treads with Polyurethane
Question posed on www.WoodworkingTalk.com. My answer follows here’s the query:
Hello All,

I have a small problem and wanted to ask some advice.

I purchased a new set of treads and risers for my stairway.

I just finishing staining and putting 3 coats of floor grade Polyurethane finish on all of it (satin). I used a good quality natural brush for this DIY. The polyurethane is oil based.

For the most part they look good. I sanded with 220 grit between coats and each coat had less bubbles than the last and was even more smooth.

The problem is that even after the 3 coats there are a couple of air bubles here and there (maybe 4-5 small ones per tread). I’m not sure if that’s normal or if there is a way to do a last coat (maybe a different method) to make it all smooth.

I was thinking of a very light final sanding, diluting some polyurethane with mineral spirits and rubbing on a final coat with a lint free cloth (or similar).

Any suggestions? Should I just sand the areas and wipe on a bit in those areas where the bubbles are or just leave it alone because it’s normal.

I can’t say I’ve really taken a close look at stained stairs at other peoples houses so I just don’t know.

When I went shopping to see how much they were already stained and finished, all the samples were flawless, but maybe the makes sure that all samples are perfect.

Any help would be appreciated.

Here’s my thoughts:

The problem of bubbles often starts with the way you use the product.
To start with the product:
Stir the product, don’t shake. Leave to settle 10 mins.
Apply sealing coat, thin finish no more than 10%.
Don’t over load brush, run with grain laying on the finish
Don’t scrub
Over lap strokes, keeping wet edge.
Once a coat has been layed, I then go over area with
lightly loaded brush to blend in.
Once dry, use 320 zinc stearate paper. Run along in direction of grain.
Rub between sucessive coats with 320 or 400.
When satisfied with finish (no less than 3) if needed,
De nib with 600 wet dry, I’ve started using orange oil as lubricant seems
to work for me.

Hope this helps.

Good luck,