Polyurethane, How many coats?
by Howard Acheson

Question:   jrmbadger
How many coats of poly do people usually use?

I'm in the process of building a bookcase and have 2 coats of poly on already...   I usually use 2 coats, but is there a benefit of a third or fourth coat?

Thanks in advance, J

Answers:   Howard Acheson
Two full strength coats is enough for an item that will get minimal abuse.   A kitchen table would get three generally.

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Sorry, "full strength coats" means brushed on, thinned only 10-15% maximum.

If you use a wiping varnish, you need to wipe on 2-3 coats to equal one brushed on coat.

Of course, with something as absorbent as MDF, your first coat will be almost completely absorbed.   So an additional coat may be in order.

My normal brushing schedule is to apply the first coat thinned 50%.   Then I do a second coat thinned 10-15%.   A tabletop will get an additional coat.   If you are going to "finish the finish" a further coat is a good idea.

Finally, the more poly varnish you put on, the more "plastic" it will look and the longer you will have to let it cure before it can be "rubbed".

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For example - " 1 coat, then 2 coats .... then add additional coats as needed."

I really don't see too much wrong with those directions.   They do suppose that you have a "look" in mind.   "As needed" would be for such things as a surface that gets lots of abuse (kitchen tabletop, hall table that has keys thrown on it, etc), when you want to "finish the finish" or when you want to fill the pores and have a perfectly flat surface.

Finishing is not an exact thing and the manufacturer can't know exactly what you want to do.   The instructions get you to the point where you have what the manufacturer feels is a satisfactory finish.   The user has to bring something to the party too.   Good finishing takes practise, experience and even some skill.   You don't just pick up a brush and do it.   Remember, the manufacturer wants the user to have a good experience with the product so they spend some time with instructions on how best to use the product.

If you want some good info about finishes and finishing, I recommend "Understanding Wood Finishing" by Flexner.


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