Should 1/16 inch gaps between boards be filled with wood filler?

Q: I have a newly unfinished ash wood floor with 4 1/4 planks. I want to use a water based polyurethane finish – 2 coats of sealer and 2 of finish. In the winter there are small (1/16th”) gaps between some of the boards when the heat is on. Should I use a full trowel filler, water based wood filler? With 4 coats of sealer on it, will the filler pop out in the summer when the humidity goes up?

A: Given the width of the planks 1/16 gap is minuscule.  You must be pleased with that.  I wouldn’t use wood filler on this at all.  I would check to make sure the sealer you use is designed to prevent side bonding.  Filler never stays in if there is any kind of movement in the board.

Follow-up: Can you please tell me the best time of year to seal the wood floor. If it is sealed when expanded in the summer, will the sealer  reduce the seasonal contraction issue? Is it better to seal in the winter when the humidity is low and the heat is on so the sealer would run into the cracks between the boards. I’m using Street Shoe water based polyurethane. Thank you so much for your help.

A: Your floor brings to mind a fact regarding ash which can be important where wood is used for heating.  Ash is the only species where you can cut down a tree and burn it soon after without stacking it for a year or more so it can dry.  Ash doesn’t hold much water.  This probably helps explain why the few gaps in your floor are very tiny.  So, in your situation I don’t think there is an advantage in picking a season to finish ash.  For other species, and in general, I can see the value of doing the finishing during more humid seasons when the furnace is not running.  Otherwise, what can happen, especially with solvent based coatings which take longer to dry is the finish will seep between the boards where they don’t receive much fresh air to help them dry.  It can stay soft for a very long time.  When humid weather comes and the boards expand it can push this soft finish to the surface of the floor.  It is a condition called poly beads.  Of course, this can be technically very difficult work.  Using a solvent, oil based finish in heating season helps the finish to dry faster.  That is great.  However, when using a fast drying coating like Street Shoe which is difficult to work with at the best of times, the last thing you need is for it to dry faster.  But then very humid conditions can also affect these finishes, causing a cloudy look because of the moisture in the air.  I somehow survived over 45 years in this trade. 

Follow-up: Thank you so much for your expertise. I hope I am understanding correctly. Since I am using a quick drying finish, the best time to seal the floor would be in the spring when the temperature is in the 70’s. I could use a dehumidifier in order to keep the humidity at about 50% until the finish is good and dry. Does this sound okay? 

A: Yes, but be sure there is no air blowing across the floor while applying the finish or until it has started to set up.

Follow-up: Okay, I understand. I am curious why you said Street Shoe finish is difficult to apply. Is it because of the quick drying? Is Bona easier?

A: Yes, it set up very quickly.  This was many years ago.  Maybe things have changed since then.

Follow-up: Street Shoe came highly recommended. It does require an added catalyst. I plan on 2 coats of sealer and 2 of finish. This will be my first time applying a water based finish. I know it is completely different from the oil based finishes I am very familiar with. I am aware of the fast drying issue and the potential humidity problems. I have the appropriate applicators and have watched videos on application. They make it look easy, but I’m sure it’s not. I am apprehensive about leaving streaks or overlap marks. Do you have any tips or pointers? 

A: You have to move the finish across the room quickly.  If it is a square room you might do ok.  If there are closets and hallways, well I wish you the best.  Don’t shake the finish.

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